[evanlinjin]

Description

Standardizes the PR description's changelog notice so release managers can scrape it instead of hand-collecting it.

The format is one fenced changelog block in the PR description:

```changelog
### Fixed
- bdk_esplora, bdk_electrum: `full_scan` no longer stops early when ... #1234
```

• headings are keepachangelog's (Added, Changed, Deprecated, Removed, Fixed, Security), as each crates/*/CHANGELOG.md already uses;
• each entry is one unwrapped line - <crate>[, <crate>...]: <change>, ending with #PR (the scraper adds it if missing);
• PRs with nothing to log leave the block out.

Changes:

• CONTRIBUTING.md documents the format, and the PR template shows it (inside its HTML comment, so an untouched template has no block).
• ci/changelog.py check validates the notice of a PR description on stdin: known headings, entry syntax, crate names (bdk_<dir> for each crates/<dir>), one block at most, no empty block. A PR without a block passes.
• .github/workflows/changelog.yml runs it on PR open/edit/push. The body goes through env, not the script text, and the job has read-only permissions.
• python3 ci/changelog.py scrape <rev-range> [<crate>] (e.g. chain-0.23.2..master bdk_chain) finds the Merge bitcoindevkit/bdk#N commits in the range, fetches each PR's description with gh, and prints per-crate keepachangelog sections, warning about malformed notices.

Not included: the github-merge prompt suggested in the issue, since that script lives outside this repo.

Fixes #2025

Changelog notice

None: CI and contributor docs only.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.66%. Comparing base (7b68d28) to head (1366909).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2425   +/-   ##
=======================================
  Coverage   80.66%   80.66%           
=======================================
  Files          31       31           
  Lines        6236     6236           
  Branches      293      293           
=======================================
  Hits         5030     5030           
  Misses       1136     1136           
  Partials       70       70           

Flag	Coverage Δ
rust	80.66% <ø> (ø)

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