Replace sqlite3 subprocess with bun:sqlite native driver, add AGENTS.md and tests

Rewrites history/queries.ts to use bun:sqlite with readonly mode and
parameterized queries instead of spawning sqlite3. Consolidates threshold
constants into a single source of truth. Adds AGENTS.md as the canonical
operational doc, moves architecture to docs/, and adds initial test suite.

package.json

@@ -40,4 +40,4 @@
     "@types/node": "^20.14.0",
     "typescript": "^5.7.3"
   }
-}
+}