Fix setup cache path and webhook CLI entrypoint

[melonelish]

Summary

• replace the author-local absolute uv cache path with repository-local .uv-cache
• ignore .uv-cache/ in .gitignore
• add the documented top-level finchbot webhook command
• share Webhook startup logic between finchbot webhook and finchbot channel serve
• add CLI regression tests for Webhook command registration

Tests

• uv run pytest -q