feat(tag): add [tag] mode = bump-only (#17)

[tizz98]

Summary

Adds a [tag] mode = "full" | "bump-only" config to meta tag, the mechanism that reconciles meta tag with a CI-owns-tag release.yml (issue #17).

• full (default) is unchanged: bump + commit + create + push the tag.
• bump-only: bump + commit + push the branch, but do not create or push a tag — a CI workflow (release.yml) creates the tag on merge and builds + publishes. This removes the silent no-op where meta tag pushing its own tag made release.yml's prepare job skip build + publish (which shipped nothing for v0.4.0).

Scope note — this repo's adoption is deferred

This PR ships the feature only (parser + command + tests). It does not switch ai-meta's own .meta/meta.toml to bump-only, because CI runs the pinned ./meta shim (.meta/version = 0.4.0), and that released binary — with deny_unknown_fields — can't parse a [tag] table it predates. A repo's meta.toml can only use fields the pinned release already understands, so dogfooding [tag] has to wait until a release ships this parser and .meta/version is bumped. Tracked in #20 (and the underlying self-hosting-CI gap in #21).

How it works

• A TagMode enum is resolved onto EffectiveConfig.tag_mode, parsed once in config::load_from_str and threaded through merge (mirroring the ProfileKind precedent). An invalid mode string is a hard load error, not a silent fall back to full — a silent fallback would re-introduce the bug.
• meta tag derives a pure, unit-tested ReleasePlan { create_tag, push_tag, enforce_branch } from the mode and branches its git orchestration on it. In bump-only: no tag created/pushed, and the "must be on main" branch guard is relaxed (the release PR is cut from a feature branch). Full mode is byte-identical to before.
• Composes with Implement configurable lifecycle hooks in meta.toml #16 (lifecycle hooks): once Implement configurable lifecycle hooks in meta.toml #16 lands, a post_tag hook can run gh pr create for full automation. No hook logic in this PR.

Test plan

• cargo test --workspace — config parse (full / bump-only / invalid) + both ReleasePlan selections. (process::tests::which_finds_sh fails on the Windows sandbox only — no sh on PATH; passes on Linux CI.)
• cargo clippy --all-targets -- -D warnings clean; cargo fmt --all -- --check clean.
• ./meta check --strict (pinned shim, as CI runs it) — passes.
• cargo run -- tag patch --dry-run with a local [tag] mode = "bump-only" shows [bump-only] + the bump-only note and no branch refusal off main.

Follow-ups

• Adopt [tag] mode = "bump-only" in ai-meta's own meta.toml (post-release) #20 — adopt [tag] mode = "bump-only" in ai-meta's own .meta/meta.toml + update docs/releasing.md, after the release that ships this parser (and .meta/version bump).
• Self-hosting CI can't validate ai-meta's own meta.toml with its in-tree binary #21 — self-hosting CI can't validate ai-meta's own meta.toml with its in-tree binary (the pinned-shim gap that forced the deferral above).

Addresses #17 (provides the reconciliation mechanism); this repo's adoption tracked in #20.

🤖 Generated with Claude Code