fix: guard worktree audits and Pi agent restoration - #241

Worktree audits could mark a worktree safe when default transcript discovery silently discarded an unreadable runtime directory. Pi session restore could mark an agent stopped before its previous process exited, allowing a second process to resume the same session and working directory.

• Keep transcript roots when inspection fails for reasons other than absence. The audit reports the scan failure and leaves recent activity unknown, so the worktree requires review.
• Keep restored agents unavailable until the previous process is confirmed gone. Only identity-matched orphans are signalled; older records without enough identity data refuse continuation while the process remains alive.
• Add regression coverage for unreadable roots, discovery errors, delayed orphan termination, and legacy records, including successful continuation after exit. Update the Pi tool reference to describe the behavior.

Validation:

• Repository suite: 1,189 passed, 36 skipped.
• Orchestration and watch-PR suites: 225 passed.
• Signed-in Pi integration suite using real processes and model calls: 11 passed.
• Pi and script TypeScript checks, script formatting, generated-file checks, and diff checks passed.

The queued-PR cancellation fix found during the same review is already merged in #239 and is part of this PR's base.