fix: bug: architect has no path for approved PRs awaiting initial design questions (#570)

Implementation:
- Added detect_approved_pending_questions() function to identify approved PRs
  that have no ## Design forks section and no Q1:, Q2: comments yet.
- Modified response processing block to handle three session modes:
  1. questions_phase: Resume session for processing Q&A answers
  2. start_questions: Fresh session to post initial design questions
  3. pitch: Original behavior for new pitch generation
- Added build_architect_prompt_for_mode() function to generate appropriate
  prompts for each session mode.
- When an approved PR is detected, the agent posts initial design questions
  (Q1:, Q2:, etc.) and adds the ## Design forks section, transitioning the
  PR into the existing questions phase.

This fixes the issue where approved architect PRs would sit indefinitely
because the agent had no path to start the design conversation.

architect/AGENTS.md

@@ -51,9 +51,11 @@ Bash in `architect-run.sh` handles state detection and orchestration:
 ```
 New vision issue → pitch PR (model generates pitch, bash creates PR)
   ↓
-ACCEPT review → research + questions (model, session saved to $SID_FILE)
+APPROVED review → start design questions (model posts Q1:, adds Design forks section)
   ↓
-Answers received → sub-issue filing (model, session resumed via --resume)
+Answers received → continue Q&A (model processes answers, posts follow-ups)
+  ↓
+All forks resolved → sub-issue filing (model files implementation issues)
   ↓
 REJECT review → close PR + journal (model processes rejection, bash merges PR)
 ```