ci: centralize release versioning with independent change fragments

[zhanghanduo]

Problem

Every published-code PR currently edits the same version, lockfile and changelog header. Merging one PR immediately conflicts with its peers even when their code changes are independent.

Change

Feature PRs add independent changes/<id>.fix|feature|breaking.md files and keep the current release version. scripts/prepare_release.py aggregates fragments into one release PR, selects PATCH or MINOR, updates the version/changelog and consumes the fragments; uv lock completes preparation.

The existing version-bump status name is retained for branch protection. CI checks new fragments, protects existing notes from deletion outside releases, validates release versions/notes/lock consistency, and no longer permits the old label bypass. Tag validation rejects pending fragments or a stale project lock. Development snapshots are identified by commit SHA; package versions identify releases.

The same shared commit is included in #42, #43 and #40 so those PRs use the new gate immediately. Their only release metadata changes are independent fragments. Merge this workflow PR first for a smaller subsequent review; it is also safe if a migrated feature PR merges first.

Validation

• Release workflow tests cover fragment gates, aggregation, version selection, tag guards and merging independent feature branches.
• Ruff passes.
• All six merge orders of fix(observers): make TaskBoardObserver critical so its reminder lands #42, feat(api): export helpers that host facades re-export #43 and fix(loop): make text-mode tool_call ids unique across the history #40 complete without conflicts in isolated worktrees.