fix: persist failed headless transcripts

[Brian Krabach (bkrabach)]

Summary

• persist the accumulated headless transcript when execution fails before the normal save path
• preserve existing session metadata and emit the normal store lifecycle events
• keep persistence failures from masking the original execution error

Non-goals

• no changes to interactive-session persistence
• no changes to durable checkpoints, session ownership, or takeover behavior
• no provider behavior or context-limit changes

Validation

• Local suite: 2,422 passed, 1 skipped, 13 deselected, 1 expected xfail
• DTU supplemental validation: 2,419 passed, 4 skipped, 13 deselected, 1 expected xfail
• PTY validation: 13 passed
• New regression coverage: 25 passed; base comparison had 12 expected failures
• Real Context/Loop/root-writer path verified that a tool-result failure persists before cleanup and restores in a fresh process
• Synthetic offline provider only; no live provider validation
• Independent final review approved the reviewed diff

PR CI and post-merge CI remain required. No host installations.