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. Resolved SHAs are validated
before use to prevent index-out-of-range panics.

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

docs/runtimes.md

@@ -44,7 +44,7 @@ The sandbox has two key directories that map to Claude Code's config levels:
 /sandbox/
 ├── claude-config/                   ← CLAUDE_CONFIG_DIR (personal level)
 │   ├── agents/
-│   │   └── <name>.md                   Agent definition (filename derived from AgentName())
+│   │   └── <name>.md                   Agent definition (filename derived from DerivedName())
 │   ├── skills/
 │   │   ├── code-review/SKILL.md        Built-in skills (personal level — wins on collision)
 │   │   ├── pr-review/SKILL.md