v2.6.0: Auto-recovery + date search fix + production hardening

Fixes

• Auto-recovery from corrupted database: search.js, index.js, and boot.js now detect SQLITE_NOTADB/SQLITE_CORRUPT and auto-rebuild the index instead of crashing with an unhandled exception.
• Date-only search: Queries like "4月2日" now correctly return all content from April 2nd. Previously returned empty results because date tokens (月/日) were searched as literal text in chunks.
• Date scope: Date-only queries no longer include undated files (MEMORY.md).

Improvements

• memory-boot.js retries with --force rebuild if initial index update fails
• SKILL.md documents boot.js in Commands section

Verified

• Full clean-install simulation (empty workspace → all features working)
• Corrupted DB recovery tested (search/index/boot all recover)
• 31 test scenarios passed including: empty workspace, concurrent writes, large files, special characters, SQL injection, date filtering, CJK/English/mixed search