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/plugin/tool-registry.ts

@@ -94,7 +94,10 @@ export function createToolRegistry(args: {
     getSessionID: getSessionIDForMcp,
   })
 
-  const commands = discoverCommandsSync(ctx.directory)
+  const commands = discoverCommandsSync(ctx.directory, {
+    pluginsEnabled: pluginConfig.claude_code?.plugins ?? true,
+    enabledPluginsOverride: pluginConfig.claude_code?.plugins_override,
+  })
   const skillTool = createSkillTool({
     commands,
     skills: skillContext.mergedSkills,