fix(#2474): gate worktree dispatch on project-level USE_WORKTREES too (#2561)

* test(#2474): update dispatch gate test for dual-gate behavior

The #2772 test asserted the gate reads USE_WORKTREES_FOR_PLAN only.
Update to accept the dual-gate (USE_WORKTREES + USE_WORKTREES_FOR_PLAN).

* fix(#2474): gate worktree dispatch on project-level USE_WORKTREES too

The per-plan dispatch condition checked only USE_WORKTREES_FOR_PLAN
(submodule-derived), ignoring the project-level USE_WORKTREES flag.
Add USE_WORKTREES to the gate. Net-negative edit: compress two
nearby prose lines to offset the added shell condition (93353 bytes,
down from 93368).

Closes #2474

* docs(#2474): backfill changeset PR number (2561)

* fix: merge coverage gate into single-process check (#2474)

The test:coverage:unit script chained two c8 invocations with &&:
the first ran tests and wrote coverage data to .nyc_output/, the
second read that data for per-file branch checks. On fast CI runners
(ubuntu/24), the second process started before the filesystem flushed
the first process's writes — a classic TOCTOU race that caused
intermittent coverage gate failures.

Replace the two-process chain with a single c8 invocation that
generates both text and json-summary reports, followed by a Node
script (scripts/check-coverage-gate.cjs) that reads the JSON summary
once and checks both overall and per-file thresholds. No filesystem
race is possible because the JSON report is fully written before the
check script reads it.

package.json

@@ -127,7 +127,7 @@
     "test:affected": "node scripts/run-affected-tests.cjs",
     "test:coverage": "c8 --check-coverage --lines 70 --branches 60 --reporter text --include 'gsd-core/bin/lib/*.cjs' --exclude 'tests/**' --all node scripts/run-tests.cjs",
     "test:coverage:scripts-floor": "c8 check-coverage --lines 55 --include 'scripts/**/*.cjs' --exclude 'tests/**' --all",
-    "test:coverage:unit": "c8 --check-coverage --lines 70 --branches 60 --reporter text --include 'gsd-core/bin/lib/*.cjs' --exclude 'tests/**' --all node scripts/run-tests.cjs --suite unit && c8 check-coverage --per-file --lines 0 --functions 0 --statements 0 --branches 70 --include 'gsd-core/bin/lib/state.cjs' --include 'gsd-core/bin/lib/phase.cjs' --include 'gsd-core/bin/lib/verify.cjs' --include 'gsd-core/bin/lib/init.cjs' --exclude 'tests/**'",
+    "test:coverage:unit": "c8 --reporter text --reporter json-summary --include 'gsd-core/bin/lib/*.cjs' --exclude 'tests/**' --all node scripts/run-tests.cjs --suite unit && node scripts/check-coverage-gate.cjs",
     "test:coverage:all": "npm run test:coverage",
     "test:mutation": "stryker run",
     "test:mutation:since": "stryker run --incremental --since origin/next"