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>

tests/test_plane_status_model.py

@@ -141,6 +141,7 @@ def test_tc23_qg_checks_registry_unchanged():
         "check_reviewer_verdict", "check_tests_local", "check_deploy_status",
         "check_staging_status", "check_branch_mergeable", "check_staging_image_fresh",
         "check_security_gate",  # ORCH-022 integ: security-gate registered
+        "check_coverage_gate",  # ORCH-027 integ: coverage-gate registered
     }