feat(cli): add fullsend agent subcommand for managing agent registrations

Implements ADR 0058 Phase 2: CLI commands for managing agent entries in
fullsend config files (both org and per-repo).

Subcommands:
- agent add: register by URL or local path with SHA pinning and integrity hash
- agent list: display registered agents in a table
- agent update: re-pin URL agents to a new commit SHA
- agent remove: remove agents and clean up unused allowlist prefixes

GitHub blob/tree URLs are automatically converted to raw content URLs.
Non-GitHub URLs require a pinned commit SHA. Forge client is injected
via dependency injection for testability.

Signed-off-by: Greg Allen <gallen@redhat.com>
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Greg Allen <gallen@redhat.com>

Author: ggallen