One test proves cross-forge following end to end: parent commits, child checks, take pull opens, owner merges #2659

The follow path across forges landed in pieces (#2631, #2633, #2649, #2651, #2656), each with its own unit tests. Nothing runs the whole path as a person lives it, so a seam between the pieces can break unseen.

Done looks like:

• [ ] A new test/follow-e2e.test.ts builds a parent forge and a child whose parent.json points at it, both with the fixtures test/spawn-chain.test.ts already uses (loopback ports or local bare paths, no network). The steps:

1. The parent commits twice. 2. The child's checkUpstream runs by hand (no timers): behind is 2, and the tell-me card lists 2 commits. 3. The mode is switched to take-updates, and the next check opens one take pull whose body lists the 2 commits, with the default branch unchanged. 4. The pull is merged through the existing merge action, and the next check says up to date. 5. A conflicting parent commit opens one issue and no pull.

• [ ] It runs in under 10 s, and it is named in GATE in scripts/gate.ts.
• [ ] No production code changes unless the test finds a real seam. Any fix is named in the pull.