fix: add workaround for Claude Code classifyHandoffIfNeeded bug (#480)

Claude Code v2.1.27+ has a bug where all Task tool agents report
"failed" due to `classifyHandoffIfNeeded is not defined` — a function
called but never defined in the cli.js bundle. The error fires AFTER
all agent work completes, so actual work is always done.

This adds spot-check fallback logic to execute-phase, execute-plan,
and quick workflows: when an agent reports this specific failure,
verify artifacts on disk (SUMMARY.md exists, git commits present).
If spot-checks pass, treat as successful.

Tracked upstream: anthropics/claude-code#24181

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

get-shit-done/workflows/execute-phase.md

@@ -160,7 +160,11 @@ Execute each wave in sequence. Within a wave: parallel if `PARALLELIZATION=true`
    - Bad: "Wave 2 complete. Proceeding to Wave 3."
    - Good: "Terrain system complete — 3 biome types, height-based texturing, physics collision meshes. Vehicle physics (Wave 3) can now reference ground surfaces."
 
-5. **Handle failures:** Report which plan failed → ask "Continue?" or "Stop?" → if continue, dependent plans may also fail. If stop, partial completion report.
+5. **Handle failures:**
+
+   **Known Claude Code bug (classifyHandoffIfNeeded):** If an agent reports "failed" with error containing `classifyHandoffIfNeeded is not defined`, this is a Claude Code runtime bug — not a GSD or agent issue. The error fires in the completion handler AFTER all tool calls finish. In this case: run the same spot-checks as step 4 (SUMMARY.md exists, git commits present, no Self-Check: FAILED). If spot-checks PASS → treat as **successful**. If spot-checks FAIL → treat as real failure below.
+
+   For real failures: report which plan failed → ask "Continue?" or "Stop?" → if continue, dependent plans may also fail. If stop, partial completion report.
 
 6. **Execute checkpoint plans between waves** — see `<checkpoint_handling>`.
 
@@ -320,6 +324,7 @@ Orchestrator: ~10-15% context. Subagents: fresh 200k each. No polling (Task bloc
 </context_efficiency>
 
 <failure_handling>
+- **classifyHandoffIfNeeded false failure:** Agent reports "failed" but error is `classifyHandoffIfNeeded is not defined` → Claude Code bug, not GSD. Spot-check (SUMMARY exists, commits present) → if pass, treat as success
 - **Agent fails mid-plan:** Missing SUMMARY.md → report, ask user how to proceed
 - **Dependency chain breaks:** Wave 1 fails → Wave 2 dependents likely fail → user chooses attempt or skip
 - **All agents in wave fail:** Systemic issue → stop, report for investigation