A tell through the API with variants: 2 starts two builders whose runs and pulls share one group id #1571

Replaces #1409, made smaller: the API path only. The tell form and the MCP variants come after.

Done looks like:

• [ ] A named migration adds a nullable variant_group to pulls and runs.
• [ ] POST tell in the API accepts variants (an integer 1–4, validated at the boundary; 0, 9, "2" and 2.5 are a 400 with a message). n > 1 dispatches n builder runs with one group id, and the pull each one opens carries it.
• [ ] The pull JSON carries variantGroup and the sibling pull numbers (empty for an ordinary pull).
• [ ] Tests in the existing tell/API test file, or a new file added to GATE in scripts/gate.ts (both earlier pulls went red on exactly that): variants: 2 makes two runs in one group; 0 and 9 are refused; a plain tell has no group.

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 #1574 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 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 #1634 closed).

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

1. 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 #1574 closed)
2. 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 #1634 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.

Split into #1643 (the variant group column and the pull JSON) and #1644 (variants on the tell API, waiting on #1643). Both tries went red in test/asks-expiry.test.ts and test/asks-rank.test.ts because they changed how green pulls fold into asks; both new issues keep asks untouched and name those suites to run. #1410 (the deck that chooses among siblings) now waits for #1644.