test: plan-mode handshake E2E coverage and unit assertions

Adds 6 E2E test files and 8 new unit assertions to verify the plan-mode
handshake works end-to-end and stays correct under regeneration.

E2E tests (gate-tier, paid, EVALS=1 EVALS_TIER=gate):
- test/skill-e2e-plan-ceo-plan-mode.test.ts — handshake fires before any
  Write/Edit when plan-mode distinctive phrase is present; 2-option shape
  (Exit/Cancel); option A routes to ExitPlanMode cleanly
- test/skill-e2e-plan-eng-plan-mode.test.ts — same contract for plan-eng
- test/skill-e2e-plan-design-plan-mode.test.ts — same contract for
  plan-design; exercises C-cancel branch instead of A-exit
- test/skill-e2e-plan-devex-plan-mode.test.ts — same contract for plan-devex
- test/skill-e2e-plan-mode-no-op.test.ts — negative regression: handshake
  must NOT fire when distinctive phrase is absent; skill proceeds normally
  through Step 0 (REGRESSION RULE guardrail against breaking existing
  interactive-review sessions)
- test/e2e-harness-audit.test.ts — free unit test asserting every
  `interactive: true` skill has at least one canUseTool-using test file
  (prevents future drift where a skill opts in without coverage)

Shared helper test/helpers/plan-mode-handshake-helpers.ts centralizes the
canUseTool interceptor + distinctive-phrase injection so the 4 sibling
E2E tests are thin wiring (~20 LOC each) and can't drift out of sync.

Unit assertions added to test/gen-skill-docs.test.ts:
- handshake section present in all 4 Claude-generated SKILL.md files
- handshake section absent from non-interactive Claude skills (ship,
  review, qa, office-hours, codex, retro, cso)
- handshake section absent from non-Claude host outputs (.agents, etc.)
- 0C-bis STOP block present in plan-ceo-review/SKILL.md at correct
  position (between the "Present these approach options" line and
  "### 0D-prelude" header)
- handshake resolver wired BEFORE generateUpgradeCheck in preamble
  composition order

6 new gate-tier entries added to test/helpers/touchfiles.ts so any change
to the handshake resolver, preamble composition, skill templates, question
registry, one-way-door classifier, or agent-sdk-runner fires the relevant
E2E tests. test/touchfiles.test.ts updated for the new selection count
(plan-ceo-review/** now triggers 15 tests, up from 8).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

test/helpers/touchfiles.ts

@@ -82,6 +82,17 @@ export const E2E_TOUCHFILES: Record<string, string[]> = {
   'plan-eng-review-artifact':  ['plan-eng-review/**'],
   'plan-review-report':        ['plan-eng-review/**', 'scripts/gen-skill-docs.ts'],
 
+  // Plan-mode handshake (v1.10.2.0) — gate-tier safety regression tests.
+  // Each fires when any of: the interactive skill's template, the resolver,
+  // preamble composition, the Agent SDK harness, the question registry, or
+  // the one-way-door classifier changes.
+  'plan-ceo-review-plan-mode':    ['plan-ceo-review/**', 'scripts/resolvers/preamble/generate-plan-mode-handshake.ts', 'scripts/resolvers/preamble.ts', 'scripts/question-registry.ts', 'scripts/one-way-doors.ts', 'test/helpers/agent-sdk-runner.ts'],
+  'plan-eng-review-plan-mode':    ['plan-eng-review/**', 'scripts/resolvers/preamble/generate-plan-mode-handshake.ts', 'scripts/resolvers/preamble.ts', 'scripts/question-registry.ts', 'scripts/one-way-doors.ts', 'test/helpers/agent-sdk-runner.ts'],
+  'plan-design-review-plan-mode-handshake': ['plan-design-review/**', 'scripts/resolvers/preamble/generate-plan-mode-handshake.ts', 'scripts/resolvers/preamble.ts', 'scripts/question-registry.ts', 'scripts/one-way-doors.ts', 'test/helpers/agent-sdk-runner.ts'],
+  'plan-devex-review-plan-mode':  ['plan-devex-review/**', 'scripts/resolvers/preamble/generate-plan-mode-handshake.ts', 'scripts/resolvers/preamble.ts', 'scripts/question-registry.ts', 'scripts/one-way-doors.ts', 'test/helpers/agent-sdk-runner.ts'],
+  'plan-mode-no-op':              ['plan-ceo-review/**', 'scripts/resolvers/preamble/generate-plan-mode-handshake.ts', 'scripts/resolvers/preamble.ts', 'test/helpers/agent-sdk-runner.ts'],
+  'e2e-harness-audit':            ['plan-ceo-review/**', 'plan-eng-review/**', 'plan-design-review/**', 'plan-devex-review/**', 'scripts/resolvers/preamble/generate-plan-mode-handshake.ts', 'test/helpers/agent-sdk-runner.ts'],
+
   // AskUserQuestion format regression (RECOMMENDATION + Completeness: N/10)
   // Fires when either template OR the two preamble resolvers change.
   'plan-ceo-review-format-mode':      ['plan-ceo-review/**', 'scripts/resolvers/preamble/generate-ask-user-format.ts', 'scripts/resolvers/preamble/generate-completeness-section.ts', 'scripts/resolvers/preamble.ts', 'model-overlays/opus-4-7.md'],
@@ -317,6 +328,14 @@ export const E2E_TIERS: Record<string, 'gate' | 'periodic'> = {
   'plan-eng-coverage-audit': 'gate',
   'plan-review-report': 'gate',
 
+  // Plan-mode handshake — deterministic safety regression, gate-tier
+  'plan-ceo-review-plan-mode': 'gate',
+  'plan-eng-review-plan-mode': 'gate',
+  'plan-design-review-plan-mode-handshake': 'gate',
+  'plan-devex-review-plan-mode': 'gate',
+  'plan-mode-no-op': 'gate',
+  'e2e-harness-audit': 'gate',
+
   // AskUserQuestion format regression — periodic (Opus 4.7 non-deterministic benchmark)
   'plan-ceo-review-format-mode': 'periodic',
   'plan-ceo-review-format-approach': 'periodic',