fix: preserve release Gate state through Part C

[Gandy2025]

Summary

Fix the release Gate lifecycle handoff so Part C exercises the exact payload,
HOME, and working directory qualified by Part B.

• Treat L1–L7 PASS as the per-provider entry condition for Part C.
• Preserve the installed payload, HOME, and CWD across the case-required
  provider sessions in G1–G6.
• Run L8 immediately on any stop before L7 PASS, or after the provider's
  Part C run completes or stops.
• Keep the final D0 requirement unchanged: both providers need L1–L8 and
  G1–G6 PASS.
• Add deterministic regression coverage for the sequencing contract and
  receipt labels.

Scope

Included: the manual Gate lifecycle, live-case entry/teardown instructions,
receipt labels, and deterministic contract coverage.

Excluded: product skill behavior, model-backed Gate execution, repository
visibility/settings, tags, GitHub Releases, and publishing.

Validation

• Skill structure validation — Python 3.10 and 3.12
• Full unit test suite — 91 tests on each Python version; one expected
  case-sensitive-filesystem skip on macOS
• Deterministic Floor — all 5 cases PASS on each Python version
• Python compileall — Python 3.10 and 3.12
• Ruff lint and format checks
• Agent Skills validation
• Pinned installer discovery — skills@1.5.22
• skills@latest discovery smoke
• git diff --check

Commands:

python3.10 scripts/validate_skills.py
python3.10 -m unittest discover -s tests -v
python3.10 evals/floor.py
python3.10 -m compileall -q scripts skills tests evals
python3.12 scripts/validate_skills.py
python3.12 -m unittest discover -s tests -v
python3.12 evals/floor.py
python3.12 -m compileall -q scripts skills tests evals
uvx ruff check .
uvx ruff format --check .
uvx --from skills-ref agentskills validate skills/run-with-first-tree
npx --yes skills@1.5.22 add . --list
npx --yes skills@latest add . --list
git diff --check

Independent Standards and Spec reviews found lifecycle edge cases during the
pre-commit review. Those findings were fixed and both final reviews reported
no remaining findings.

Safety and compatibility

• No credentials, personal data, model transcripts, or live-run artifacts
• English canonical docs and Chinese mirrors are synchronized when affected
  — no Chinese mirror exists for these Gate files
• Support and release claims do not exceed the evidence collected
• Breaking changes and migration steps are documented — not applicable;
  this repairs the previously unsatisfiable Gate sequence

The PR remains Draft because independent QA was explicitly excluded from this
change. A successor release candidate still requires a fresh, separately
authorized live Gate; deterministic checks do not qualify agent behavior.