Revisit E3's early 'cancelled' write now that G4 wired the planning endpoints #156

Problem

docs/implementation/runner-lifecycle.md, decision 2, keeps E3 writing cancelled before the provider settles (a read-only-phase exception) and says: "Revisit this when G4 wires the planning endpoints." G4 merged in PR #152, but no PR or document records that revisit. A reader cannot tell whether the exception still stands.

Acceptance

• Decide whether E3's early cancelled write stays, now that the browser drives drafts and suggestions through F's production endpoints (G2–G4).
• Record the decision and reason in decision 2 of docs/implementation/runner-lifecycle.md.
• If the behaviour changes, add a regression that fails under the old behaviour.

Found during the Lane G close-out review (#145).