Promote aitools skills-management commands to top-level

[jamesbroadhead]

Summary

Promotes the aitools skills-management surface out of experimental/ so the stable half lives at databricks aitools … and slots in next to the other top-level command groups. The matching interface changes (--scope enum, --project/--global deprecation, --agents auto-detect doc) live in a stacked follow-up: #5234.

This is mostly a move, but it is not move-only — see Non-move changes below.

• Source files for install, update, uninstall, list, version (and the agents/installer libs they depend on) physically move from experimental/aitools/ to cmd/aitools/ + libs/aitools/, matching the existing cmd/apps/ + libs/apps/ layout. OWNERS, Taskfile, and the pr-checklist skill are updated to match.
• The top-level command is registered at databricks aitools ….
• Keeps the tools subtree under experimental/aitools/ — query, discover-schema, get-default-warehouse, statement … — because tools.go still says "There are no stability guarantees for these tools".
• The old paths under databricks experimental aitools install/update/uninstall/list/version and databricks experimental aitools skills install/list keep working as deprecated backward-compat aliases that print a notice pointing at the new path (via cobra's Deprecated field).

The aitools skills-management surface is feature-complete after the 5-PR series (#4810–#4814) that added state tracking, lifecycle commands, and project scope support. The tools subtree is functionally useful but its shape is still in flux, so promoting only the stable half.

Non-move changes

In addition to the file moves, this PR:

• Removes the redundant aitools/README.md (apps/pipelines don't carry one); the same info lives in the command's Long description.
• Rewrites the Long description on the new top-level databricks aitools command.
• Adds a deprecation notice to every legacy alias under databricks experimental aitools (Lennart's review ask — they used to forward silently).
• Refactors how the legacy experimental aitools skills install [name] wrapper is wired: the wrapper now lives in cmd/aitools/legacy_skills.go alongside the install code it wraps, and the previously-exported test-injection vars (InstallSkillsForAgentsFn, ListSkillsFn, PromptAgentSelection) are back to unexported. The two now-empty experimental files (experimental/aitools/cmd/skills.go and skills_test.go) are deleted.

What's not in this PR

These are deliberately separated and reviewed independently:

• aitools: add --scope flag, deprecate --project/--global #5234 — --scope=project|global|both flag, deprecation of --project/--global via cobra.Deprecated, --agents auto-detect help text.
• aitools list: emit JSON via --output json #5233 (draft) — --output json on databricks aitools list.

Command shape after this PR

# Stable, top-level
databricks aitools install      # use --skills <name>[,<name>...] for specific skills
databricks aitools update
databricks aitools uninstall
databricks aitools list
databricks aitools version

# Backward-compat aliases (print deprecation notice; point at the new paths)
databricks experimental aitools install/update/uninstall/list/version
databricks experimental aitools skills {list,install}

# Experimental, unchanged path
databricks experimental aitools tools query
databricks experimental aitools tools discover-schema
databricks experimental aitools tools get-default-warehouse
databricks experimental aitools tools statement {submit,get,status,cancel}

Test plan

• databricks aitools --help shows install/update/uninstall/list/version (no tools)
• databricks --help lists aitools in the output
• databricks experimental aitools install prints a deprecation notice and still forwards
• databricks experimental aitools tools query … runs as before
• databricks experimental aitools tools --help lists query/discover-schema/get-default-warehouse/statement
• Existing aitools tests pass; the legacy-wrapper tests moved with the wrapper to cmd/aitools/legacy_skills_test.go

This pull request was AI-assisted by Isaac.

[simonfaltum]

I think we should move the code itself (the files) out of experimental as part of this as well.

[simonfaltum]

LGTM, will do a follow up to mark this as beta