P2: Add close/dispose API for SQLite store

[vamgan]

Problem

SqliteTaskStore opens a better-sqlite3 database but exposes no public close()/dispose method.

Relevant area:

• src/index.ts

Impact

Long-running processes, CLIs, MCP servers, and tests cannot reliably release file handles. This can cause flaky cleanup and unclear lifecycle management.

Suggested fix

Add a public method such as:

close(): void {
  this.db.close();
}

Document lifecycle expectations in the README and use it in tests/consumers where appropriate.

Acceptance criteria

• Store exposes a documented close/dispose method.
• Tests or examples demonstrate clean shutdown.