Update package READMEs as part of the release process

[Copilot]

The release skills and process docs had no step to keep the shared NuGet embedded README (src/PACKAGE.md) current -- no package-list closure enforcement, no badge strategy, no release-notes link. The existing snippet-validation reference also pointed to per-package README paths that don't exist.

Reference docs

• New prepare-release/references/readme-content.md -- maintenance checklist covering: single-shared-file invariant, package-list closure (non-counting phrasing), badge strategy (nuget/vpre during prerelease / nuget/v for stable), release-notes link pattern, and root README.md sync.
• Fixed prepare-release/references/readme-snippets.md -- replaced stale "Which READMEs to Validate" table (src/ModelContextProtocol.Core/README.md, src/ModelContextProtocol.AspNetCore/README.md -- neither exists) with the actual src/PACKAGE.md.

Skill updates

• prepare-release Step 9 -- expanded to run both the content checklist (closure, badges, release-notes link, root sync) and snippet validation. Added edge cases: new package introduced, release type change, forward-referencing release-notes link.
• publish-release Step 5 -- broadened from snippet-only to a full README review pass with final suggestions before the release is created. Added edge cases: stale closure, wrong badge style, missing/incorrect release-notes link.

Process docs

• release-process.md -- notes the README update work in both the prepare and publish sections.

Live illustrative fix to src/PACKAGE.md

This change does not affect the already-published 2.0.0-preview.1 package. It illustrates the README updates that will apply for 2.0.0-preview.2+.

• Added ModelContextProtocol.Extensions.Apps to the package list (was omitted, including from its own embedded README on nuget.org).
• Rewrote "This SDK consists of three main packages" to "The SDK packages are:" -- avoids a stale count when packages are added.
• Switched all four package badges from nuget/v to nuget/vpre -- nuget/v renders nothing during a prerelease-only series.
• Added a release-notes link to https://github.com/modelcontextprotocol/csharp-sdk/releases/tag/v2.0.0-preview.1.

Root README.md mirrors the package-list closure change (Extensions.Apps added, non-counting phrasing); its badges remain nuget/v since that view is GitHub, not nuget.org.

Original prompt

Task: Keep NuGet embedded package READMEs current as part of the release process

Goal

The release process (.github/release-process.md) and the prepare-release /
publish-release skills are missing a step to refresh the NuGet embedded
README for each release. Add that step to the skills + process docs, fix a stale
reference doc, and apply an illustrative fix to the live shared README.

Make all changes on a new branch and open a pull request against main titled
Update package READMEs as part of the release process.

Essential codebase facts (verify before editing)

• All four packages share a single embedded README: src/PACKAGE.md. Each
  csproj packs it via <None Include="..\PACKAGE.md" Pack="true" PackagePath="\README.md" />
  with <PackageReadmeFile>README.md</PackageReadmeFile>. Updating src/PACKAGE.md
  updates every package's nuget.org README at once. The four packages are:
  ModelContextProtocol.Core, ModelContextProtocol,
  ModelContextProtocol.AspNetCore, and ModelContextProtocol.Extensions.Apps.
• src/PACKAGE.md currently says "This SDK consists of three main packages" and
  lists only Core / ModelContextProtocol / AspNetCore — it omits
  ModelContextProtocol.Extensions.Apps (including not listing itself in its own
  embedded README on nuget.org).
• Badges in src/PACKAGE.md use shields.io nuget/v/... which always render the
  latest stable version (currently 1.4.0), not the prerelease series the current
  packages belong to.
• The root README.md (the GitHub repo readme; NOT packed into packages) carries
  the same "three main packages" closure and should be kept in sync.
• .github/skills/prepare-release/references/readme-snippets.md is stale: its
  "Which READMEs to Validate" table lists src/ModelContextProtocol.Core/README.md
  and src/ModelContextProtocol.AspNetCore/README.md, which DO NOT EXIST. The only
  packed README is src/PACKAGE.md.
• Current version in src/Directory.Build.props: <VersionPrefix>2.0.0</VersionPrefix>
  • <VersionSuffix>preview.1</VersionSuffix> → 2.0.0-preview.1.

Confirmed decisions

• Badge strategy: during a prerelease series use shields.io nuget/vpre badges
  (latest including prereleases); for stable releases use nuget/v. Document both.
• Release-notes link: add ONE statement to the shared src/PACKAGE.md linking to
  https://github.com/modelcontextprotocol/csharp-sdk/releases/tag/v{version} (the
  same link for all packages, since the README is shared). The tag will not exist at
  prepare time but resolves by publish time.
• Fix the live src/PACKAGE.md now, illustratively. The PR description MUST state
  that this change does NOT affect the already-published 2.0.0-preview.1 package; it
  illustrates what the README updates will look like for 2.0.0-preview.2+.
• Correct the stale readme-snippets.md reference to target src/PACKAGE.md.

Changes to make

1. New reference: .github/skills/prepare-release/references/readme-content.md

Create a maintenance checklist for the shared embedded README covering:

• The single-shared-src/PACKAGE.md invariant (one file → every package README).
• Package-list closure: every shipping SDK package must be listed (including any
  newly introduced package, and including the package in its own README). Reword the
  "three main packages" phrasing into a non-counting closure so adding a package
  doesn't leave a stale count.
• Badge strategy: nuget/vpre during a prerelease series, nuget/v for stable;
  apply consistently to all package badges.
• Release-notes link: a statement linking to
  https://github.com/modelcontextprotocol/csharp-sdk/releases/tag/v{version}.
• Root README.md sync: keep the repo readme's package-list closure aligned.
• Other salient new data: new package descriptions, getting-started pointers, etc.

2. Fix .github/skills/prepare-release/references/readme-snippets.md

Replace the stale "Which READMEs to Validate" table (Core/AspNetCore per-package
READMEs that do not exist) with the actual packed file src/PACKAGE.md. Update the
"What to Extract" / source-file guidance so snippet validation extracts from
src/PACKAGE.md (and the root README.md if relevant). Keep the rest of the
validation procedure intact.

3. Update .github/skills/prepare-release/SKILL.md

In Step 9 ("Review and Update Documentation"), sub-step 1 ("NuGet package READMEs"):
require BOTH (a) the existing snippet validation AND (b) running the README-content
checklist from the new references/readme-content.md — verify package closure, apply
the prerelease/stable badge strategy, and add/update the release-notes link for
v{version}. Reference the new doc. Add Edge Cases entries, e.g.:

• "New package introduced → add it to the README package closure."
• "Release tag does not yet exist at prepare time → th...

[jeffhandley]

@copilot In addition to the suggestions posted, please also update src/ModelContextProtocol.Extensions.Apps/ModelContextProtocol.Extensions.Apps.csproj to apply a <Description> of "MCP Apps extension for building interactive UI applications that render inside MCP hosts."

[Copilot]

@copilot In addition to the suggestions posted, please also update src/ModelContextProtocol.Extensions.Apps/ModelContextProtocol.Extensions.Apps.csproj to apply a <Description> of "MCP Apps extensio...

Done in f0f1cb8 -- updated the <Description> in ModelContextProtocol.Extensions.Apps.csproj and also applied the same wording to the package entries in src/PACKAGE.md and README.md.

[jeffhandley]

This addresses issues found in the 2.0.0-preview.1 package READMEs, and augments the process.

1. We didn't add the MCP.Extensions.Apps package
2. We were linking to the stable package versions for the badges
3. We weren't linking to the release-notes from the package READMEs
4. This integrates into the release process at the same stage as bumping the version
5. It also integrates into the publishing stage for final pass validation over the content and sample code

This won't fix the 2.0.0-preview.1 READMEs that were published, but it gets the content updated and ready for 2.0.0-preview.2 to build on, illustrating where the changes will be applied going forward.