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/hooks/auto-slash-command/detector.test.ts

@@ -102,6 +102,19 @@ After`
       expect(result?.args).toBe("project")
     })
 
+    it("should parse namespaced marketplace commands", () => {
+      // given a namespaced command
+      const text = "/daplug:run-prompt build bridge"
+
+      // when parsing
+      const result = parseSlashCommand(text)
+
+      // then should keep full namespaced command
+      expect(result).not.toBeNull()
+      expect(result?.command).toBe("daplug:run-prompt")
+      expect(result?.args).toBe("build bridge")
+    })
+
     it("should return null for non-slash text", () => {
       // given text without slash
       const text = "regular text"