GET /api/v1/repos/:o/:n/pulls reads 50 pulls' merge states under 10 ms when no tip has moved #6143

Measured on 2026-10-26: GET /api/v1/repos/trevato/forge/pulls?limit=50 takes 100–128 ms, of which merge is 91–115 ms. pullMergeStates (src/core/pulls.ts:802) asks git about every pull on every request.

Done when:

• [ ] pullMergeStates caches each pull's answer by (base tip, head tip), in memory and bounded. It asks git only for pulls whose pair changed.
• [ ] A test with 50 open pulls asserts that the second call does no git work and finishes under 10 ms, and that a push to one pull's head recomputes exactly that pull.

Files: src/core/pulls.ts, test/pull-merge-states-cache.test.ts. Risk: a base that moves under every pull at once (a merge to main) recomputes them all, which is correct. Do not key the cache on the pull alone.