Support customize release title format via config

Support change `announcement_title` of `plan-dist-manifest.json` via config. Now it's extracted from changelog titles, but they're not always equal. For example I prefer `## 0.1.0 (2026-01-18)` in changelog but ` v0.1.0` in GitHub release.

Start by locating where plan-dist-manifest.json gets its announcement_title from changelog titles and where release configuration is parsed. Trace the generated title for a versioned changelog, then make the configuration control it independently; done means changelog and GitHub release titles can differ as described.

Written by the indexing model from the issue text.