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.

opencode.json

@@ -1,4 +1,4 @@
 {
   "plugin": ["@alkdev/open-memory"],
   "$schema": "https://opencode.ai/config.json"
-}
+}