fix(#2661): unconditional plan-checkbox sync in execute-plan (#2682)

* fix(#2661): unconditional plan-checkbox sync in execute-plan

Checkpoint A in execute-plan.md was wrapped in a "Skip in parallel mode"
guard that also short-circuited the parallelization-without-worktrees
case. With `parallelization: true, use_worktrees: false`, only
Checkpoint C (phase.complete) then remained, and any interruption
between the final SUMMARY write and phase complete left ROADMAP.md
plan checkboxes stale.

Remove the guard: `roadmap update-plan-progress` is idempotent and
atomically serialized via readModifyWriteRoadmapMd's lockfile, so
concurrent invocations from parallel plans converge safely.

Checkpoint B (worktree-merge post-step) and Checkpoint C
(phase.complete) become redundant after A is unconditional; their
removal is deferred to a follow-up per the RCA.

Closes #2661

* fix(#2661): gate ROADMAP sync on use_worktrees=false to preserve single-writer contract

Adversarial review of PR #2682 found that unconditionally removing the
IS_WORKTREE guard violates the single-writer contract for shared
ROADMAP.md established by commit dcb50396 (PR #1486). The lockfile only
serializes within a single working tree; separate worktrees have
separate ROADMAP.md files that diverge.

Restore the worktree guard but document its intent explicitly: the
in-handler sync runs only when use_worktrees=false (the actual #2661
reproducer). Worktree mode relies on the orchestrator's post-merge
update at execute-phase.md lines 815-834, which is the documented
single-writer for shared tracking files.

Update tests to assert both branches of the gate:
- use_worktrees: false mode runs the sync (the #2661 case)
- use_worktrees: true mode does NOT run the in-handler sync
- handler-level idempotence and lockfile contention tests retained,
  scope clarified to within-tree concurrency only

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

@@ -402,15 +402,19 @@ If SUMMARY "Issues Encountered" ≠ "None": yolo → log and continue. Interacti
 </step>
 
 <step name="update_roadmap">
-**Skip this step if running in parallel mode** (the orchestrator handles ROADMAP.md
-updates centrally after merging worktrees).
+Run this step only when NOT executing inside a git worktree (i.e.
+`use_worktrees: false`, the bug #2661 reproducer). In worktree mode each
+worktree has its own ROADMAP.md, so per-plan writes here would diverge
+across siblings; the orchestrator owns the post-merge sync centrally
+(see execute-phase.md §5.7, single-writer contract from #1486 / dcb50396).
 
 ```bash
-# Auto-detect parallel mode: .git is a file in worktrees, a directory in main repo
+# Auto-detect worktree mode: .git is a file in worktrees, a directory in main repo.
+# This mirrors the use_worktrees config flag for the executing handler.
 IS_WORKTREE=$([ -f .git ] && echo "true" || echo "false")
 
-# Skip in parallel mode — orchestrator handles ROADMAP.md centrally
 if [ "$IS_WORKTREE" != "true" ]; then
+  # use_worktrees: false → this handler is the sole post-plan sync point (#2661)
   gsd-sdk query roadmap.update-plan-progress "${PHASE}"
 fi
 ```