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.

biome.json

@@ -1,5 +1,5 @@
 {
-  "$schema": "https://biomejs.dev/schemas/2.3.11/schema.json",
+  "$schema": "https://biomejs.dev/schemas/2.4.12/schema.json",
   "vcs": {
     "enabled": true,
     "clientKind": "git",
@@ -17,4 +17,4 @@
       "recommended": true
     }
   }
-}
+}