One call rebuilds combined/N for every open kept-fork pull into the default branch #10053

I added recombine(forge, repoId) in src/core/recombine.ts, and nothing calls it yet. The four test files you named pass (9 tests) and bun x tsc --noEmit is clean. I did not run bun scripts/gate.ts, scripts/fast.ts or bun scripts/design-lint.ts (design-lint because nothing under src/web/ changed). I did not check whether they exist in this clone.

What it does

• Which pulls: it reads the default branch's tip once at the start of a pass, then calls combine for each open pull from a kept fork into that branch. Each call uses the pull's refs/pull/N/head.
• One pass at a time: it runs one pass per repository. A call made during a pass sets one "again" flag, and exactly one more pass runs when the first ends. The returned count is the passes the run took, so 1 or 2, and a call that joins a running pass gets that run's count.
• Failures: a pull that fails to combine (a missing ref, a combine that returns nothing, or an exception) is logged as a warning and skipped. The other pulls are still rebuilt.

Tests (test/combined-recombine.test.ts)

• After main moves and recombine runs, combined/N has the new main as a parent, for two fork pulls.
• A pull whose head ref is deleted is logged and left alone, and the other pull still follows main.
• Three concurrent calls run exactly two passes, so never more than two.

Left out and one deviation

• forge argument: recombine takes { db, reposRoot, log }, a subset of App, so the real app can be passed as is.
• Test setup: the new test builds a real app, as combined-fork.test.ts does, instead of reusing combined-git.test.ts's fixtures. recombine reads pulls from the database, which a git-only fixture does not have.
• Hook: nothing is wired to a push hook.
• Not retried: a pull that fails is skipped for that pass and not retried.

Closes #10039

I added recombine(forge, repoId) in src/core/recombine.ts, and nothing calls it yet. The four test files you named pass (9 tests) and bun x tsc --noEmit is clean. I did not run bun scripts/gate.ts, scripts/fast.ts or bun scripts/design-lint.ts (design-lint because nothing under src/web/ changed). I did not check whether they exist in this clone.

What it does

• Which pulls: it reads the default branch's tip once at the start of a pass, then calls combine for each open pull from a kept fork into that branch. Each call uses the pull's refs/pull/N/head.
• One pass at a time: it runs one pass per repository. A call made during a pass sets one "again" flag, and exactly one more pass runs when the first ends. The returned count is the passes the run took, so 1 or 2, and a call that joins a running pass gets that run's count.
• Failures: a pull that fails to combine (a missing ref, a combine that returns nothing, or an exception) is logged as a warning and skipped. The other pulls are still rebuilt.

Tests (test/combined-recombine.test.ts)

• After main moves and recombine runs, combined/N has the new main as a parent, for two fork pulls.
• A pull whose head ref is deleted is logged and left alone, and the other pull still follows main.
• Three concurrent calls run exactly two passes, so never more than two.

Left out and one deviation

• forge argument: recombine takes { db, reposRoot, log }, a subset of App, so the real app can be passed as is.
• Test setup: the new test builds a real app, as combined-fork.test.ts does, instead of reusing combined-git.test.ts's fixtures. recombine reads pulls from the database, which a git-only fixture does not have.
• Hook: nothing is wired to a push hook.
• Not retried: a pull that fails is skipped for that pass and not retried.