Short sessions that exit cleanly below SAVE_INTERVAL and without a PreCompact
were never saved. Add a SessionEnd hook that takes one final flush.
Claude Code budgets SessionEnd hooks at 1.5s and a plugin-provided timeout
cannot raise it, and a cold mempalace start exceeds that, so the wrapper
backgrounds the work and returns immediately; the detached child completes the
transcript ingest, project mine, and diary checkpoint after the session exits.
The handler validates transcript_path through _validate_transcript_path before
any ingest or diary write, so a traversal or wrong-suffix path is rejected while
the independent project mine still runs.
Adds hook_session_end, both shell wrappers, the plugin hooks.json entry, the
session-end CLI choice, and focused tests.
(cherry picked from commit 10e1450e04fc7cec72984ab7442d3b4fca1490e8)
mvalentsev