fix(annotator): use shared fetchWithRetry in CourtListener client

[williamzujkowski]

Closes #223.

Changes

• Adopts fetchWithRetry from @civic-source/shared in CourtListener client.
• Removes duplicated private fetchWithRetry loop and sleep helper from packages/annotator/src/client.ts.
• Adds signal?: AbortSignal parameter to searchByStatute for cancellation and timeout support.
• Preserves CourtListener-specific 401 token authentication mapping.
• Adds test coverage in client.test.ts verifying immediate abort on cancellation without retrying.

Closes #223 item 2, resolving the last open item in #223.