feat(dispatch): wire marketplace plugin commands into slash command dispatch

Connect the existing plugin loader infrastructure to both slash command
dispatch paths (executor and slashcommand tool), enabling namespaced
commands like /daplug:run-prompt to resolve and execute.

- Add plugin discovery to executor.ts discoverAllCommands()
- Add plugin discovery to command-discovery.ts discoverCommandsSync()
- Add "plugin" to CommandScope type
- Remove blanket colon-rejection error (replaced with standard not-found)
- Update slash command regex to accept namespaced commands
- Thread claude_code.plugins config toggle through dispatch chain
- Add unit tests for plugin command discovery and dispatch

Closes #2019

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Codex <noreply@openai.com>

src/tools/skill/tools.test.ts

@@ -464,7 +464,7 @@ describe("skill tool - ordering and priority", () => {
     const tool = createSkillTool({ skills, commands })
 
     //#then: should include priority info
-    expect(tool.description).toContain("Priority: project > user > opencode > builtin")
+    expect(tool.description).toContain("Priority: project > user > opencode > builtin/plugin")
     expect(tool.description).toContain("Skills listed before commands")
   })