fix: load bun:sqlite at runtime with node:sqlite fallback

[thatdudealso]

Summary

• src/storage/db.ts statically imported bun:sqlite, which only resolves under the Bun runtime. This blocked running storage tests under plain Node (e.g. node node_modules/.bin/vitest run), since bunx vitest run itself still executes under Bun.
• Loads bun:sqlite lazily via createRequire and falls back to Node's built-in node:sqlite (DatabaseSync) when Bun isn't available. Both backends satisfy the same internal Database shape, so the public SQLiteDatabase interface and all callers are unchanged.
• Adds src/storage/db.test.ts, a regression test that initializes the database and round-trips a write/read (including inside withTransaction), proving the storage layer now works under Node/Vitest.

Fixes #346

Test plan

• bun run typecheck
• bun run lint
• bun run format
• bunx vitest run (Bun runtime) — 250/250 tests pass
• node node_modules/.bin/vitest run (plain Node runtime) — 250/250 tests pass, confirming the node:sqlite fallback path works end-to-end

---

Note

Medium Risk
Touches core persistence and parameter binding on experimental node:sqlite; behavior must stay aligned with Bun’s strict mode for all existing callers.

Overview
Storage SQLite no longer statically imports bun:sqlite, so Vitest and other tooling can run under plain Node. db.ts loads Bun’s driver via createRequire when available and otherwise uses node:sqlite (DatabaseSync) behind the same internal Database shape, leaving the public SQLiteDatabase API unchanged.

The Node path enables bare named parameter binding ({ id } → @id) via setAllowBareNamedParameters(true) and implements transactions with explicit BEGIN/COMMIT/ROLLBACK. New src/storage/db.test.ts covers init/read-write, named params, the Node-only spy on setAllowBareNamedParameters, and withTransaction. AGENTS.md documents the dual-runtime behavior and test commands; CLAUDE.md points agents at AGENTS.md.

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