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

docs/CONVENTIONS.md

@@ -129,3 +129,6 @@ let conn = Connection::open(path)
 - Push via `nix develop --command git push` (for prek hooks)
 - Branch naming: `feat/<name>`, `fix/<name>`, `docs/<name>`
 - Commit messages: conventional commits (`feat:`, `fix:`, `refactor:`, `docs:`)
+- Changelog: a PR that changes what users or agents see adds one entry file,
+  `changes/unreleased/<branch-name>.md`, rather than editing `CHANGELOG.md`, so
+  stacked PRs never conflict. See [changes/README.md](../changes/README.md).