feat: add BLOCKED + DIAGNOSE default rule and blocker report template (closes #552)

- Updated llm-project-workflow/SKILL.md with universal BLOCKED + DIAGNOSE rule, covered blocker classes, explicit prohibition of unsafe fallbacks, and proof section.
- Added templates/blocked-diagnose-report.md (standard template with all required fields).
- Updated workflows/work-issue.md, review-merge-pr.md, create-issue.md and templates/start-issue.md, review-pr.md, merge-pr.md, recover-bad-state.md to require and reference the rule at load.
- Updated docs/llm-workflow-runbooks.md to reinforce the rule for controller prompts and all workflows.
- All changes in isolated worktree branches/issue-552-blocked-diagnose. No root mutation. No unsafe fallbacks used.
- Tests (workflow + cleanup proofs): 26+ passed in run.
- git diff --check prgs/master...HEAD: clean.

skills/llm-project-workflow/templates/start-issue.md

@@ -10,6 +10,7 @@ Final report schema: skills/llm-project-workflow/schemas/work-issue-final-report
 Router: skills/llm-project-workflow/SKILL.md (task mode: work-issue)
 
 Rules (llm-project-workflow):
+- **BLOCKED + DIAGNOSE default (required):** If any required step (load workflow, acquire lease, prove worktree under branches/, capability, profile, tool, instruction, preflight, etc.) cannot be performed, STOP. State BLOCKED. Use [`blocked-diagnose-report.md`](../templates/blocked-diagnose-report.md) template exactly. Only safe non-mutating recovery. Report. Do not continue or fallback.
 - No repo changes without a tracking issue. If none exists, create one first;
   if it can't be created, stop.
 - Work only in an isolated branch worktree under branches/. The main checkout