fix(session): recover interrupted JSONL appends

[lewismosciski]

Change Summary

Resuming a session whose last JSONL write was interrupted can lose the next message: appending joins it to the unfinished record, and the reader drops the combined line. A write interrupted inside a UTF-8 character also prevents the session from opening.

Separate an unterminated tail from the next append and skip records with invalid UTF-8 when reading. Keep the existing bytes and complete records intact, including a valid final record missing only its newline.

Validation: six regressions failed before the fix; 171 related tests passed afterward, with 20 skips. Tests cover partial JSON, partial UTF-8, a complete unterminated record, metadata-sidecar recovery, event readers, and sequence numbers. Applicable changed-file pre-commit hooks passed. Full-repository pre-commit reports existing main lint/format failures, including skills_manager.py:61 and shell_validator.py:91.

Related issue number

Found while inspecting session-log recovery; no existing issue.

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Run pre-commit install and pre-commit run --all-files before git commit, and passed lint check. (Both run; existing repository failures noted above.)
• Documentation reflects the changes where applicable (internal recovery fix; no API change)