extract release notes from date-headed changelog entries

[fabracht]

Summary

The release workflow at .github/workflows/release.yml extracts the GitHub Release body via:

```awk
/^## [$TAG]/{found=1; next} /^## [/{if(found) exit} found{print}
```

That pattern looks for `## [0.7.5]`-style headings, but every CHANGELOG entry in this repo uses the date-led form `## YYYY-MM-DD — mqdb- `. Net effect: every release since at least v0.7.5 has shipped with an empty release body. Confirmed via `gh release view v0.7.5 --json body` (length: 0).

This PR replaces the awk one-liner with a small Python script (heredoc inside the existing `run:` step — no new tooling) that:

1. Finds the heading containing `mqdb-cli ` and remembers its date (the release date).
2. Walks the file again and emits every section dated the same day — covering parallel crate bumps like the pending v0.7.6, which carries both `mqdb-cli 0.7.6` and `mqdb-cluster 0.3.4` dated 2026-05-06.
3. Trims trailing blank lines.
4. Falls back to an empty body (not a failure) if no matching heading is found, so a release where the CHANGELOG was forgotten still publishes binaries.

No CHANGELOG content moves; the existing date-led convention is preserved.

Test plan

• Dry-run for tag `0.7.6` against current `CHANGELOG.md` — extracts both the `mqdb-cli 0.7.6` and `mqdb-cluster 0.3.4` sections plus the "Discovered while running the new E2E" subsection.
• Dry-run for tag `0.7.5` against current `CHANGELOG.md` — extracts the `mqdb-cluster 0.3.2` and `mqdb-cli 0.7.5` sections (both dated 2026-04-25). Confirms backward extraction.
• YAML parses cleanly via `yaml.safe_load`; the heredoc closing `PY` lands at column 0 after block-scalar de-indentation.
• `cargo make format-check` — clean.
• `cargo make clippy` — clean (pedantic, all targets + wasm).
• Pre-commit hook ran on the commit and passed.