docs: update all docs for optional-skills and browse command

Update 7 documentation files to reflect:
- optional-skills/ directory in all project structure trees
- 'hermes skills browse' in all CLI command listings
- '/skills browse' in all slash command references
- Three-tier skill placement (bundled → optional → hub)
- 'official' trust level in trust level tables
- Updated /skills description from 'Search, install...' to 'Browse, search...'

Files updated:
- CONTRIBUTING.md (skill classification, project tree, section title)
- AGENTS.md (project tree, Skills Hub description, source adapters list)
- website/docs/reference/cli-commands.md (CLI table, slash command table)
- website/docs/developer-guide/creating-skills.md (structure, classification, trust)
- website/docs/user-guide/features/skills.md (hub commands, trust table, slash commands)
- website/docs/user-guide/cli.md (slash command description)
- website/docs/developer-guide/architecture.md (project tree)

website/docs/reference/cli-commands.md

@@ -71,6 +71,7 @@ These are commands you run from your shell.
 
 | Command | Description |
 |---------|-------------|
+| `hermes skills browse` | Browse all available skills with pagination (official first) |
 | `hermes skills search <query>` | Search skill registries |
 | `hermes skills install <identifier>` | Install a skill (with security scan) |
 | `hermes skills inspect <identifier>` | Preview before installing |
@@ -156,7 +157,7 @@ Type `/` in the interactive CLI to see an autocomplete dropdown.
 | Command | Description |
 |---------|-------------|
 | `/cron` | Manage scheduled tasks |
-| `/skills` | Search, install, inspect, or manage skills |
+| `/skills` | Browse, search, install, inspect, or manage skills |
 | `/platforms` | Show gateway/messaging platform status |
 | `/verbose` | Cycle tool progress: off → new → all → verbose |
 | `/<skill-name>` | Invoke any installed skill |