The fast check cannot see a pull's commit messages, so the gate's Why-now rule always fails there #8883

mergeForCheck now appends the pull's own commit messages to the merge commit it writes, so the gate's Why-now rule can see them in the fast tier. I did not run the fast tier itself, only the pieces below.

src/checks/merged.ts takes git log -n 200 --format=%B baseTip..head and adds it as a second paragraph after check of <sha> merged into <base>. If that log can't be walked, for example in a very shallow history, the paragraph is left out and the merge commit is written as before. whyNowProblems matches Why now: and Pipeline: per line, so it finds the line inside the merged message.

Checked:

• Test: the new case in test/merged-check.test.ts asserts the merge commit's body contains the pull's Why now: line. The pull's head commit in that test now carries that line, and the case fails without the change.
• Suites: bun test test/merged-check.test.ts passes (5 tests), as do bun scripts/gate.ts and bun x tsc --noEmit.
• MCP size: I did not run bun test test/mcp-size.test.ts, since no MCP tool changed.

If the fast tier's shallow fetch leaves base..head unreachable, the messages won't reach the check. I couldn't test that end to end here, so look at the first pull that adds a feature file under the fast tier.

Closes #8882

mergeForCheck now appends the pull's own commit messages to the merge commit it writes, so the gate's Why-now rule can see them in the fast tier. I did not run the fast tier itself, only the pieces below.

src/checks/merged.ts takes git log -n 200 --format=%B baseTip..head and adds it as a second paragraph after check of <sha> merged into <base>. If that log can't be walked, for example in a very shallow history, the paragraph is left out and the merge commit is written as before. whyNowProblems matches Why now: and Pipeline: per line, so it finds the line inside the merged message.

Checked:

• Test: the new case in test/merged-check.test.ts asserts the merge commit's body contains the pull's Why now: line. The pull's head commit in that test now carries that line, and the case fails without the change.
• Suites: bun test test/merged-check.test.ts passes (5 tests), as do bun scripts/gate.ts and bun x tsc --noEmit.
• MCP size: I did not run bun test test/mcp-size.test.ts, since no MCP tool changed.

If the fast tier's shallow fetch leaves base..head unreachable, the messages won't reach the check. I couldn't test that end to end here, so look at the first pull that adds a feature file under the fast tier.