docs: keep a CHANGELOG.md, with one entry file per PR

Release notes were written by hand on GitHub at tag time, so the contents
of a stacked release had to be pieced together from every PR, and output
changes that scripts and agents rely on were easy to miss.

- CHANGELOG.md backfills every published release (0.2.0 to 0.4.0) from its
  GitHub notes, verbatim.
- A PR adds changes/unreleased/<branch-name>.md instead of editing
  CHANGELOG.md, so stacked PRs never conflict. changes/README.md gives the
  sections, including Output Changes for JSON, MCP and error-code changes.
- scripts/changelog.sh: `check` validates entries and CHANGELOG.md,
  `release X.Y.Z` folds the entries into a new section (ordered by PR), and
  `notes X.Y.Z` prints a section as GitHub release notes.
- The release workflow sets the GitHub notes from the tag's section.
- New flake check `changelog` runs scripts/changelog-test.sh; both scripts
  are shellchecked.
- Entries for 0.5.0: #73, #74, #75, #78, #79, #80, #81 and this PR.

Claude-Session: https://claude.ai/code/session_01RqPPVeVDrnLoP7NUZmLRxY

README.md

@@ -193,6 +193,7 @@ User docs, MCP setup per agent, and agent-readable `llms.txt`:
 - [docs/OWNERSHIP_PUBLISH.md](docs/OWNERSHIP_PUBLISH.md): maintainer snapshot publishing
 - [docs/SMOKE.md](docs/SMOKE.md): test layout and the live provider drift check
 - [ROADMAP.md](ROADMAP.md): planned work
+- [CHANGELOG.md](CHANGELOG.md): what each release changed; unreleased entries in [changes/](changes/README.md)
 
 ## Development