test(W7h): retire the paid first-task-scaffold E2E duplicate

W7h retirement (t12 F12). Removes the case from test/helpers/touchfiles-data.ts
(E2E_TOUCHFILES, E2E_TIERS periodic, E2E_KINDS rule) and both
scripts/paid-test-durations.json entries; test/helpers/periodic-exclude-data.ts
had no entry. e2e-tier-alignment, eval-kinds, eval-detach-timeout-floor,
periodic-exclude-policy, touchfiles and paid-shards tests stay green.

Retirement card:
- test: test/skill-e2e-first-task-scaffold.test.ts ('first-task-scaffold',
  periodic, ~$0.02): Haiku is told to run exactly one detector command and the
  test greps the tool result for code_node / greenfield.
- detects: detector bucket regressions in bin/gstack-first-task-detect.
- non_test_callers: none.
- search_command: grep -rn "first-task-scaffold" test scripts .github
- stronger_proof / replacement: test/preamble-first-task-scaffold.test.ts
  (free; executes the real detector on fixture repos for every bucket,
  including code_node and greenfield, plus first-run gating and telemetry in
  bin/gstack-skill-start). The model added nothing: the prompt dictated the
  exact command.
- history: c118e24 v1.64.1.0.
- validation: bun test test/preamble-first-task-scaffold.test.ts plus the
  registration tests above.

Control (scratch worktree, against the replacement): making the detector emit
the wrong bucket for a Node project or a commit-less repo turns
preamble-first-task-scaffold.test.ts red (2/2). Log:
.context/mutation-first-task-scaffold.md.

scripts/paid-test-durations.json

@@ -18,7 +18,6 @@
       "test/skill-e2e-design.test.ts#plan-design-review-no-ui-scope": 24431,
       "test/skill-e2e-diagram.test.ts": 29998,
       "test/skill-e2e-docsync-spawned.test.ts": 137929,
-      "test/skill-e2e-first-task-scaffold.test.ts": 1000,
       "test/skill-e2e-gbrain-roundtrip-local.test.ts": 1000,
       "test/skill-e2e-hermetic-canary.test.ts": 8813,
       "test/skill-e2e-investigate-owned-completion.test.ts": 45178,
@@ -143,7 +142,6 @@
       "test/skill-e2e-design.test.ts#design-html-slop-gate": 158970,
       "test/skill-e2e-design.test.ts#plan-design-review-plan-mode": 300166,
       "test/skill-e2e-diagram.test.ts": 55187,
-      "test/skill-e2e-first-task-scaffold.test.ts": 8563,
       "test/skill-e2e-gbrain-roundtrip-local.test.ts": 1000,
       "test/skill-e2e-health.test.ts": 188584,
       "test/skill-e2e-hermetic-canary.test.ts": 1000,