Tasks: end-to-end PDLC scenario tests

Derived from design.md and testing-plan.md. Ticket: #217.

Task list

• [x] 1. Runner core (cli/tests/test_pdlc_e2e/runner.py)
  • Scenario.load (manifest parse + validation with file/field-naming refusals), ScenarioRun (tmp checkout, harness config, FakeIntegration at integrations.resolve, event log to tmp, session registry seed), the step interpreter (comment, emit, complete, advance, ask, reply, inner-loop, fail-github, restore-github, expect), the trace collector and assert_trace with first-divergence reporting.
  • Depends on: none
  • Requirements: R1.1–R1.4, R2.1–R2.2, R3.1–R3.3
  • Test: T1, T2 (red→green while authoring the happy path)
• [x] 2. Happy-path scenario (fixtures + expected trace)
  • scenarios/happy-path/ — locked spec-chain fixtures, execution-log fixture with the review-chain sections, full expected node/label/event trace.
  • Depends on: 1
  • Requirements: R1.3, R2.3 (happy path)
  • Test: T1
• [x] 3. Trivial-tier + ask-reply scenarios
  • Declared-skip checklist reply; skip provenance assertions. Ask/park/reply with session-missing fail-closed check mid-run.
  • Depends on: 2
  • Requirements: R2.3
  • Test: T1
• [x] 4. Error + loop-prevention scenarios
  • gate-rejection, review-rejection, gh-unreachable, loop-prevention.
  • Depends on: 2
  • Requirements: R2.3, R3.3, Security 2
  • Test: T1, T8
• [x] 5. Test module + meta-tests + README
  • cli/tests/test_pdlc_e2e_integration.py (named tests with Gherkin docstrings, scenario-dir↔test consistency check, runner meta-tests), README.md documenting the fixture-set format.
  • Depends on: 1–4
  • Requirements: R2.1, R2.2, R4.1, R4.3
  • Test: T2, T3
• [x] 6. Docs
  • docs/capabilities/testing-and-contracts.md (current behaviour + history row); execution-log Documentation/Capability docs sections.
  • Depends on: 1–5
  • Requirements: R4.2
  • Test: T12, T14
• [x] 7. Verification
  • Execute the plan, tick activities with evidence under evidence/.
  • Depends on: 1–6
  • Requirements: all
  • Test: the plan itself