[test-recorder] PR flow: unchecked git stash and cleanup can misreport recovery state

### Problem
The PR-creation flow's git orchestration doesn't check its own recovery steps:

- `src/pr/gh.ts:258-281,283-306` — `git stash push` result is unchecked before committing on a dirty tree; isolation may silently not exist.
- `src/pr/gh.ts:230-236,274-280,291-304` — cleanup reports "You're back on your original branch" even when checkout, stash pop (possible conflicts), or branch deletion failed; the user may actually be on the temp branch with conflicted files.

### Suggested fix
Abort before commit unless stash succeeds and the stash object is identified; check every recovery command; stop destructive follow-ups after a failure; report the actual branch/stash state with exact recovery commands.

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