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

changes/unreleased/stockbit-corpaction.md

@@ -0,0 +1,5 @@
+### New Features
+
+- `idx stocks actions` and `get_corporate_actions` list one stock's corporate actions from your Stockbit session, newest first: cash and stock dividends, bonus shares, rights issues, warrants, splits and reverse splits, shareholders' meetings, public exposes and tender offers. `--kind` picks kinds, and `--kind ipo` adds the stock's IPO. (#78)
+- `idx stocks calendar` and `get_corporate_calendar` list the market's actions dated in a range, oldest first: by default today and the next 30 days, at most 366 days. (#78)
+- An action's `date` is its cum date where there is an entitlement (dividends, bonuses, rights, splits), else the event's first day. Where Stockbit gives only an ex date, the table marks it `(ex)`. Dividends paid in another currency carry `currency`. (#78)