Add bank deletion support

[josharsh]

Summary

Banks can be created with createBank() and listed with listBanks(), but there's no way to delete a bank and all its associated data.

What needs to happen

1. Add a deleteBank(bankId: string) method to the Engram class
2. It should delete all facts, entities, relations, and observations scoped to that bank
3. Clean up FTS5 index entries for deleted facts
4. Remove the bank row itself
5. Prevent deletion of the default bank
6. Add the method to MemoryStore first, then expose through Engram

Files to look at

• src/store/store.ts — MemoryStore class, see existing createBank() and deleteFact() patterns
• src/engram.ts — Engram orchestrator, exposes store methods
• src/store/schema.ts — table definitions to understand relationships
• tests/unit/store.test.ts — add tests here

Acceptance criteria

• engram.deleteBank('my-bank') removes all scoped data
• Throws if trying to delete the default bank
• FTS5 index is cleaned up
• Unit test covering create → populate → delete → verify empty