PR opener should inject the Fixes #N line from feature metadata — 3 team PRs shipped without it, including one where it was an explicit acceptance criterion

[mabry1985]

The pattern (3 strikes)

Three team PRs have now merged (or nearly merged) without their Fixes #N line, breaking issue auto-close:

• protoAgent#2121 — first observed; issue needed manual closing; added "check the body" to the operator merge checklist
• protoAgent#2163 — dropped again; operator repaired the body pre-merge
• protoAgent#2166 — dropped even though the dispatch prompt carried "PR body carries Fixes #2165" as an explicit numbered acceptance criterion

Strike 3 is the tell: this can't be fixed by prompting the coder harder. The PR body is generated boilerplate (## Summary\n<title> (bd-xxx)\nSee the diff for details.) — the coder's criterion compliance never reaches it.

Proposed fix — inject it mechanically at PR-open

The loop already knows the linkage: the feature description carries the source GitHub issue URL (the standard dispatch format leads with it). At the PR-opening step:

1. Extract the first https://github.com/<owner>/<repo>/issues/<n> from the feature's description (or, better, add an optional explicit source_issue field to board_create_feature and prefer it when set).
2. If the issue's <owner>/<repo> matches the PR's target repo, append \n\nFixes #<n> to the generated body. If it's a different repo (plugin PR referencing a protoAgent issue, etc.), append Refs <full-url> instead — cross-repo Fixes closes issues in the other repo only with write perms and is usually not what's wanted.
3. If the coder's own body already contains a Fixes/Closes #<n> for that issue, don't duplicate.

Keeps the coder out of the loop entirely for the one line that's pure metadata, and the operator merge-checklist item becomes a no-op instead of a recurring repair.

LOE ~2: one extraction helper + one body-assembly change + tests (same-repo, cross-repo, already-present, no-issue cases).

[mabry1985]

Shipped in #95 (merged as 75b471e). Fittingly, this PR was opened by the pre-fix code and is the last one whose body needed a manual Fixes-line repair — every PR the loop opens from now on stamps its own.