[test-recorder] Prefilled --pr fails open: recording silently continues on the wrong branch

### Problem
A prefilled `--pr` fails open (`src/commands/record.ts:146-223,270-279,347-355`): PR lookup failure, dirty checkout, user refusal, or checkout failure all continue recording on the *current* branch. An agent-driven invocation can claim to target a PR while actually recording against unrelated code. The later "recording on branch X" message is informational, not a verified invariant.

### Suggested fix
Make checkout preparation return a typed outcome (requested vs effective revision) with an explicit policy: prefilled/non-interactive `--pr` fails closed unless the effective checkout matches; the interactive path may explicitly choose the fallback.

_Line references are against branch `cb/comfy-test-usability` (PR #15837); adjust after merge._