A pull's check runs on its head merged into today's base, and records the base it checked #1178

Part of #1133 (P0 Trust): a pull's green must be the merge's green.

Done looks like.

• [ ] src/checks/service.ts checks a temporary merge of the pull's head into its base's current head (never written to a branch), and the run records baseOid beside commitOid.
• [ ] A pull that does not merge runs no check and says so (the line from the conflicts issue).
• [ ] A push to the default branch is checked as today, on its own commit.
• [ ] test/checks.test.ts: a pull whose head is green alone but red merged with a newer base reads red.
• [ ] The MCP's reading of a pull's check names the base it was checked against.
• [ ] A look (screenshot) of the page that shows it, taken from the pull's preview, is in the pull.
• [ ] A test that goes red in this pull is fixed in this pull, whoever's it is.

Files. src/checks/service.ts, src/git/, a migration for baseOid, test/checks.test.ts.

Risks. The sealed-pull path (pullDir) and forks that silently followed (~line 500) must keep their current behaviour; the export must not leave the merge in the bare repo.

Fits. Mason.

Before pushing. bun x tsc --noEmit and the test files named here, quoted in the pull. One pull, green on its first push.

One pillar: this serves exactly the one it is linked under. Done means reachable on the page, through MCP, in a test and by eye (#334).

Parked by the crew: the check failed ("asks that earn their place > five rows by default, the rest one tap away; a phone shows two (#883)") and the decider saw no point in another try (pull #1536 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Parked by the crew: the check failed ("asks that expire, hold and collapse > two green pulls in one repository are one row, and every surface counts it once") and the decider saw no point in another try (pull #1573 closed).

Parked twice since a person last spoke here, so it is the planner's now. Every reason:

1. the check failed ("asks that earn their place > five rows by default, the rest one tap away; a phone shows two (#883)") and the decider saw no point in another try (pull #1536 closed)
2. the check failed ("asks that expire, hold and collapse > two green pulls in one repository are one row, and every surface counts it once") and the decider saw no point in another try (pull #1573 closed)

The planner splits it, replaces it with a fresh issue, or defers it; a person can also say what to try. Nothing starts here until then, and the board lists it.

Replaced by #1585. Main already checks a pull merged into its base's tip (5d955dc8, src/checks/merged.ts). What is left is recording the base it checked against and naming it on the pull page and in the MCP. Both tries went red on home and asks tests this change never touched.