feat: add opencode-skill-loader with 4-source priority system (#331)

* feat: add opencode-skill-loader with 4-source priority system

- Create new opencode-skill-loader feature module independent from Claude Code
- Support 4 source paths with priority: opencode-project > project > opencode > user
  - .opencode/skill/ (opencode-project)
  - .claude/skills/ (project)
  - ~/.config/opencode/skill/ (opencode)
  - ~/.claude/skills/ (user)
- Support both SKILL.md and {SKILLNAME}.md file patterns
- Maintain path awareness for file references (@path syntax)

* feat: integrate opencode-skill-loader into main plugin

- Import and use new skill loader functions
- Load skills from all 4 sources and merge into config.command
- Also merge pluginComponents.skills (previously loaded but never used)

* feat: add skill discovery to slashcommand tool

- Import and use discoverAllSkills from opencode-skill-loader
- Display skills alongside commands in tool description and execution
- Update formatCommandList to handle combined commands and skills

* refactor: remove old claude-code-skill-loader

- Delete src/features/claude-code-skill-loader/ directory (was never integrated into main plugin)
- Update plugin loader import to use new opencode-skill-loader types

* docs: update AGENTS.md for new skill loader

- Update structure to show opencode-skill-loader instead of claude-code-skill-loader
- Update Skills priority order to include all 4 sources

---------

Co-authored-by: sisyphus-dev-ai <sisyphus-dev-ai@users.noreply.github.com>

src/features/AGENTS.md

@@ -19,7 +19,7 @@ features/
 │   └── env-expander.ts         # ${VAR} expansion
 ├── claude-code-plugin-loader/  # Load external plugins from installed_plugins.json
 ├── claude-code-session-state/  # Session state persistence
-├── claude-code-skill-loader/   # Load skills from ~/.claude/skills/*/SKILL.md
+├── opencode-skill-loader/      # Load skills from OpenCode and Claude paths
 └── hook-message-injector/      # Inject messages into conversation
 ```
 
@@ -30,7 +30,7 @@ Each loader reads from multiple directories (highest priority first):
 | Loader | Priority Order |
 |--------|---------------|
 | Commands | `.opencode/command/` > `~/.config/opencode/command/` > `.claude/commands/` > `~/.claude/commands/` |
-| Skills | `.claude/skills/` > `~/.claude/skills/` |
+| Skills | `.opencode/skill/` > `~/.config/opencode/skill/` > `.claude/skills/` > `~/.claude/skills/` |
 | Agents | `.claude/agents/` > `~/.claude/agents/` |
 | MCPs | `.claude/.mcp.json` > `.mcp.json` > `~/.claude/.mcp.json` |