fix(cursor): support wal mode without shm files via immutable fallback

[theonly1me]

What changed

• Introduce openCursorDatabase with fallback to immutable URI mode when read-only WAL initialization fails.
• Add unit tests for Cursor SQLite database in WAL mode without active -shm or -wal files.
• Add unit tests for reading active uncheckpointed WAL frames.

Why

Cursor operates in SQLite WAL mode and removes -shm and -wal files when idle or cleanly closed. Opening a WAL-mode database with readonly: true when -shm is absent causes SQLite statement preparation to fail with SQLITE_CANTOPEN (errno 14). Falling back to mode=ro&immutable=1 allows reading the checkpointed store without requiring shared memory initialization.

How to verify

bun test src/observe/adapters/cursorStore.test.ts
bun run check
bun run ./src/cli/index.ts learn --dry-run

Data handling

• Any new capture source has an opt-in flag and a README entry in this PR
• Everything that reaches the network passes redactSecrets
• No raw capture in a log line, an error message, or a test fixture
• A test exercises the real entry point, not just the redaction function