Update quality-playbook skill to v1.5.6 + add agent (#1402)

Rebuilds branch from upstream/staged (was previously merged from
upstream/main, which brought in materialized plugin files that
fail Check Plugin Structure on PRs targeting staged).

Changes vs. staged:
- Update skills/quality-playbook/ to v1.5.6 (31 bundled assets:
  SKILL.md + LICENSE.txt + 16 references/ + 9 phase_prompts/ +
  3 agents/ + bin/citation_verifier.py + quality_gate.py).
- Add agents/quality-playbook.agent.md (top-level orchestrator).
  name: quality-playbook (validator-compliant).
- Update docs/README.skills.md quality-playbook row description
  + bundled-assets list to v1.5.6.
- Fix 'unparseable' → 'unparsable' in quality_gate.py (5 instances;
  codespell preference, both spellings valid).

Closes the v1.4.0 → v1.5.6 update in a single clean commit on top of
upstream/staged. The preserved backup branch backup-bedbe84-pre-rebuild
(SHA bedbe848fa3c0f0eda8e653c42b599a17dd2e354) holds the prior history for reference.

skills/quality-playbook/phase_prompts/phase6.md

@@ -0,0 +1,23 @@
+{skill_fallback_guide}
+
+You are a quality engineer doing the verification phase of a quality playbook run. Phases 1-5 are complete.
+
+Read SKILL.md - the Phase 6 section ("Phase 6: Verify"). Resolve SKILL.md via the documented fallback list above; do NOT assume any single install layout. Follow the incremental verification steps (6.1 through 6.5).
+
+Step 6.1: If quality/mechanical/verify.sh exists, run it. Record exit code.
+Step 6.2: Run quality_gate.py. Locate it via the same fallback list used for SKILL.md (`quality_gate.py` sits in the same directory as SKILL.md in every install layout — e.g., `quality_gate.py`, `.claude/skills/quality-playbook/quality_gate.py`, `.github/skills/quality_gate.py`, `.cursor/skills/quality-playbook/quality_gate.py`, `.continue/skills/quality-playbook/quality_gate.py`, `.github/skills/quality-playbook/quality_gate.py`). Then run:
+  python3 <resolved_quality_gate_path> .
+Read the output carefully. For every FAIL result, fix the issue:
+- Missing regression-test patches: generate quality/patches/BUG-NNN-regression-test.patch
+- Missing inline diffs in writeups: add a ```diff block
+- Non-canonical JSON fields: fix tdd-results.json (use 'id' not 'bug_id', etc.)
+- Missing files: create them
+After fixing all FAILs, run quality_gate.py again. Repeat until 0 FAIL.
+Save final output to quality/results/quality-gate.log.
+
+Step 6.3: Run functional tests if a test runner is available.
+Step 6.4: File-by-file verification checklist (read one file at a time, check, move on).
+Step 6.5: Metadata consistency check.
+
+Append each step's result to quality/results/phase6-verification.log.
+Mark Phase 6 complete in PROGRESS.md (use the checkbox format `- [x] Phase 6 - Verify` — do NOT switch to a table).