feat: debug skill dispatch and session manager sub-orchestrator (#2154)

* feat(2148): add specialist_hint to ROOT CAUSE FOUND and skill dispatch to /gsd-debug

- Add specialist_hint field to ROOT CAUSE FOUND return format in gsd-debugger structured_returns section
- Add derivation guidance in return_diagnosis step (file extensions → hint mapping)
- Add Step 4.5 specialist skill dispatch block to debug.md with security-hardened DATA_START/DATA_END prompt
- Map specialist_hint values to skills: typescript-expert, swift-concurrency, python-expert-best-practices-code-review, ios-debugger-agent, engineering:debug
- Session manager now handles specialist dispatch internally; debug.md documents delegation intent

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* feat(2151): add gsd-debug-session-manager agent and refactor debug command as thin bootstrap

- Create agents/gsd-debug-session-manager.md: handles full checkpoint/continuation loop in isolated context
- Agent spawns gsd-debugger, handles ROOT CAUSE FOUND/TDD CHECKPOINT/DEBUG COMPLETE/CHECKPOINT REACHED/INVESTIGATION INCONCLUSIVE returns
- Specialist dispatch via AskUserQuestion before fix options; user responses wrapped in DATA_START/DATA_END
- Returns compact ≤2K DEBUG SESSION COMPLETE summary to keep main context lean
- Refactor commands/gsd/debug.md: Steps 3-5 replaced with thin bootstrap that spawns session manager
- Update available_agent_types to include gsd-debug-session-manager
- Continue subcommand also delegates to session manager

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* test(2148,2151): add tests for skill dispatch and session manager

- Add 8 new tests in debug-session-management.test.cjs covering specialist_hint field,
  skill dispatch mapping in debug.md, DATA_START/DATA_END security boundaries,
  session manager tools, compact summary format, anti-heredoc rule, and delegation check
- Update copilot-install.test.cjs expected agent list to include gsd-debug-session-manager

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/copilot-install.test.cjs

@@ -1185,6 +1185,7 @@ describe('E2E: Copilot full install verification', () => {
       'gsd-code-fixer.agent.md',
       'gsd-code-reviewer.agent.md',
       'gsd-codebase-mapper.agent.md',
+      'gsd-debug-session-manager.agent.md',
       'gsd-debugger.agent.md',
       'gsd-doc-verifier.agent.md',
       'gsd-doc-writer.agent.md',