docs: update honcho CLI reference + document plugin CLI registration (#5308)

Post PR #5295 docs audit — 4 fixes:

1. cli-commands.md: Update hermes honcho subcommand table with 4
   missing commands (peers, enable, disable, sync), --target-profile
   flag, --all on status, correct mode values (hybrid/context/tools
   not hybrid/honcho/local), and note that setup redirects to
   hermes memory setup.

2. build-a-hermes-plugin.md: Replace 'ctx.register_command() —
   planned but not yet implemented' with the actual implemented
   ctx.register_cli_command() API. Add full Register CLI commands
   section with code example.

3. memory-provider-plugin.md: Add 'Adding CLI Commands' section
   documenting the register_cli(subparser) convention for memory
   provider plugins, active-provider gating, and directory structure.

4. plugins.md: Add CLI command registration to the capabilities table.

website/docs/user-guide/features/plugins.md

@@ -83,6 +83,7 @@ Project-local plugins under `./.hermes/plugins/` are disabled by default. Enable
 |-----------|-----|
 | Add tools | `ctx.register_tool(name, schema, handler)` |
 | Add hooks | `ctx.register_hook("post_tool_call", callback)` |
+| Add CLI commands | `ctx.register_cli_command(name, help, setup_fn, handler_fn)` — adds `hermes <plugin> <subcommand>` |
 | Inject messages | `ctx.inject_message(content, role="user")` — see [Injecting Messages](#injecting-messages) |
 | Ship data files | `Path(__file__).parent / "data" / "file.yaml"` |
 | Bundle skills | Copy `skill.md` to `~/.hermes/skills/` at load time |