fix: nullish coalescing in evaluate-session config, narrow pre-compact glob, add 11 tests
- evaluate-session.js: || 10 → ?? 10 for min_session_length (0 is valid)
- pre-compact.js: *.tmp → *-session.tmp to match only session files
- 11 new tests: config loading (min=0, null, custom path, invalid JSON),
session-end update path (timestamp, template replace, preserve content),
pre-compact glob specificity, extractSessionSummary edge cases