fix: recover terminal PR inspection and orphaned Pi agents

[michael-denyer]

A closed or merged PR with a deleted base branch made ship-pr inspect return unavailable. A restored Pi orphan could remain impossible to stop or resume after a temporary process-inspection failure.

• Preserve inspection of terminal PRs using their stored base OID when the branch is gone. Open PRs still require the current base target, and cancellation still returns not-open before any mutation for terminal PRs.
• Retry orphan cleanup after identity inspection recovers, including when inspection failed at the delayed SIGKILL check. Only one termination attempt runs at a time, each signal checks process identity, and continuation remains blocked until the previous process exits.
• Add CLI regressions for closed and merged PRs and real-process regressions for both inspection-failure windows. The fixes are in separate commits.

Validation:

• bun test tests/ --timeout 30000: 1,191 passed, 36 skipped, 0 failed.
• bun test orch watch-pr in the scripts directory: 234 passed, 0 failed.
• PSTACK_PI_LIVE=1 bun test tests/pi/live.test.mjs: 11 passed, 0 failed using real Pi processes and model calls at 43149cf.
• Pi and script TypeScript checks, script formatting, generated-file checks, and diff checks passed.
• All 14 GitHub CI and security checks passed.
• The read-only ship-pr inspect --repo michael-denyer/pstack-claude --pr 196 command now returns an inspected closed record. It failed before the fix.
• Independent standards and specification reviews found no remaining issues. The reviewers also reran the delayed-escalation reproduction successfully.

No version bump; these fixes ship with the next release.