fix: mark stacked PRs so repair can find them

[lemarier]

Change

Follow-up to #37, which merged before this commit reached the branch. A stacked PR now carries a Stacked on #<lower PR> body line, and repair treats a stacked PR as stale when that lower PR has merged and its merge commit is not in the head. With squash merges and branch deletion, GitHub retargets the upper layer to main while it still carries the lower layer's original commits, so it may not show as conflicting; the body line is how a later pass finds it. The local issue coordinator job and pr-conflict-precheck.sh already follow this rule.

Validation

• just check: tests, link, and JSON checks passed.
• Tested the Stacked on #N parsing on sample PR bodies and the compare API's ahead/behind status on real commits.

https://claude.ai/code/session_01E9psw1USAWcT2Vtmcfbgsa

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-27T19:41:16.681659Z	ca43acc	PR opened

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