feat(plan-phase): chunked mode + filesystem fallback for Windows stdio hang (#2499)

* feat(plan-phase): chunked mode + filesystem fallback for Windows stdio hang (#2310)

Addresses the 2026-04-16 Windows incident where gsd-planner wrote all 5
PLAN.md files to disk but Task() never returned, hanging the orchestrator
for 30+ minutes. Two mitigations:

1. Filesystem fallback (steps 9a, 11a): when Task() returns with an
   empty/truncated response but PLAN.md files exist on disk, surface a
   recoverable prompt (Accept plans / Retry planner / Stop) instead of
   silently failing. Directly addresses the post-restart recovery path.

2. Chunked mode (--chunked flag / workflow.plan_chunked config): splits the
   single long-lived planner Task into a short outline Task (~2 min) followed
   by N short per-plan Tasks (~3-5 min each). Each plan is committed
   individually for crash resilience. A hang loses one plan, not all of them.
   Resume detection skips plans already on disk on re-run.

RCA confirmed: task state mtime 14:29 vs PLAN.md writes 14:32-14:52 =
subagent completed normally, IPC return was dropped by Windows stdio deadlock.
Neither mitigation fixes the root cause (requires upstream Task() timeout
support); both bound damage and enable recovery.

New reference file planner-chunked.md keeps OUTLINE COMPLETE / PLAN COMPLETE
return formats out of gsd-planner.md (which sits at 46K near its size limit).

Closes #2310

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

* fix(plan-phase): address CodeRabbit review comments on #2499

- docs/CONFIGURATION.md: add workflow.plan_chunked to full JSON schema example
- plan-phase.md step 8.5.1: validate PLAN-OUTLINE.md with grep for OUTLINE
  COMPLETE marker before reusing (not just file existence)
- plan-phase.md step 8.5.2: validate per-plan PLAN.md has YAML frontmatter
  (head -1 grep for ---) before skipping in resume path
- plan-phase.md: add language tags (text/javascript/bash) to bare fenced
  code blocks in steps 8.5, 9a, 11a (markdownlint MD040)
- Rejected: commit_docs gate on per-plan commits (gsd-sdk query commit
  already respects commit_docs internally — comment was a false positive)

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

* fix(plan-phase): route Accept-plans through step 9 PLANNING COMPLETE handling

Honors --skip-verify / plan_checker_enabled=false in 9a fallback path.

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

---------

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

docs/CONFIGURATION.md

@@ -43,6 +43,7 @@ GSD stores project settings in `.planning/config.json`. Created during `/gsd-new
     "plan_bounce": false,
     "plan_bounce_script": null,
     "plan_bounce_passes": 2,
+    "plan_chunked": false,
     "code_review_command": null,
     "cross_ai_execution": false,
     "cross_ai_command": null,
@@ -150,6 +151,7 @@ All workflow toggles follow the **absent = enabled** pattern. If a key is missin
 | `workflow.plan_bounce` | boolean | `false` | Run external validation script against generated plans. When enabled, the plan-phase orchestrator pipes each PLAN.md through the script specified by `plan_bounce_script` and blocks on non-zero exit. Added in v1.36 |
 | `workflow.plan_bounce_script` | string | (none) | Path to the external script invoked for plan bounce validation. Receives the PLAN.md path as its first argument. Required when `plan_bounce` is `true`. Added in v1.36 |
 | `workflow.plan_bounce_passes` | number | `2` | Number of sequential bounce passes to run. Each pass feeds the previous pass's output back into the validator. Higher values increase rigor at the cost of latency. Added in v1.36 |
+| `workflow.plan_chunked` | boolean | `false` | Enable chunked planning mode. When `true` (or when `--chunked` flag is passed to `/gsd-plan-phase`), the orchestrator splits the single long-lived planner Task into a short outline Task followed by N short per-plan Tasks (~3-5 min each). Each plan is committed individually for crash resilience. If a Task hangs and the terminal is force-killed, rerunning with `--chunked` resumes from the last completed plan. Particularly useful on Windows where long-lived Tasks may hang on stdio. Added in v1.38 |
 | `workflow.code_review_command` | string | (none) | Shell command for external code review integration in `/gsd-ship`. Receives changed file paths via stdin. Non-zero exit blocks the ship workflow. Added in v1.36 |
 | `workflow.tdd_mode` | boolean | `false` | Enable TDD pipeline as a first-class execution mode. When `true`, the planner aggressively applies `type: tdd` to eligible tasks (business logic, APIs, validations, algorithms) and the executor enforces RED/GREEN/REFACTOR gate sequence. An end-of-phase collaborative review checkpoint verifies gate compliance. Added in v1.36 |
 | `workflow.cross_ai_execution` | boolean | `false` | Delegate phase execution to an external AI CLI instead of spawning local executor agents. Useful for leveraging a different model's strengths for specific phases. Added in v1.36 |