feat(coverage): deterministic test-coverage gate on deploy-staging->deploy edge (ORCH-027)

Introduce a deterministic (no-LLM) coverage sub-gate that blocks coverage
degradation before a task branch merges into `main`. Existing gates judge only by
the FACT of passing (check_ci_green / check_tests_passed / merge-gate re-test), not
by completeness — so a batch autonomous run (ORCH-088) silently erodes coverage.

Pattern mirrors the security-gate (ORCH-022): leaf src/coverage_gate.py (never-raise)
+ thin check_coverage_gate in QG_CHECKS + _handle_coverage_gate splice in advance_stage,
run AFTER merge-gate (measured on the caught-up HEAD that lands in main) and BEFORE
image-freshness (fail before the expensive docker rebuild).

- measure_coverage: pytest --cov=src --cov-report=json in the per-branch worktree ->
  line coverage %; None on tool error -> fail-open + WARNING by default (FR-6).
- compute_coverage_verdict (pure): absolute | baseline | both + epsilon (NFR-4 anti-flap);
  baseline None -> bootstrap (absolute-only).
- coverage_baseline DB table (additive, CREATE TABLE IF NOT EXISTS) + ratchet-up in
  _handle_merge_verify (deploy->done): atomic compare-and-set under merge-lease, never
  decreases; bootstrap on first merge.
- Artefact 18-coverage-report.md (coverage_status: frontmatter, single source of truth);
  GET /queue `coverage` block; FAIL -> Telegram; optional POST /coverage/baseline override.
- Flags ORCH_COVERAGE_* (kill-switch + self-hosting-only scope) -> enduro untouched;
  STAGE_TRANSITIONS / existing check_* / verdict keys byte-for-byte unchanged (NFR-5/AC-8).
- pytest-cov==5.0.0 added to requirements.txt.

Tests: tests/test_coverage_gate.py (TC-01..TC-15). Frozen QG-registry anti-regress
tests + deploy-staging edge tests updated for the new sub-gate. Full suite green.

Docs: README / adr-0029 / PIPELINE_DOCS / 18-coverage-report.md template (architecture
stage) + CHANGELOG / CLAUDE.md / .env.example (this PR).

Refs: ORCH-027
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

requirements.txt

@@ -4,6 +4,10 @@ pydantic-settings==2.5.0
 httpx==0.27.0
 pytest==8.3.3
 pytest-asyncio==0.23.8
+# ORCH-027: coverage measurement for the coverage-gate. pytest-cov wraps coverage.py;
+# the gate runs `pytest --cov=src --cov-report=json` in the per-branch worktree and
+# reads totals.percent_covered (line coverage). Offline — no network at measure time.
+pytest-cov==5.0.0
 # ORCH-022: dependency audit (OSV/PyPI advisory) for the security-gate. Needs the
 # network at scan time -> an unreachable feed degrades fail-open + warning by
 # default (ADR-001 Р-3 / 07-infra I-2). gitleaks (secret-scan) is a pinned Go