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/README.md

@@ -0,0 +1,47 @@
+# phase_prompts/
+
+Externalized phase prompt bodies for the Quality Playbook.
+
+v1.5.4 F-1 (Bootstrap_Findings 2026-04-30) extracted these from
+`bin/run_playbook.py`'s inline string templates so both execution
+modes — UI-context skill-direct (a coding agent walking through
+SKILL.md inline) and CLI-automation runner-driven (`python -m
+bin.run_playbook`) — read from the same single source of truth.
+Without externalization the two modes drift; with it, an edit to a
+phase prompt lands once and benefits both.
+
+## File layout
+
+- `phase1.md` ... `phase6.md` — one file per pipeline phase. Loaded
+  by `bin/run_playbook.py::_load_phase_prompt`.
+- `single_pass.md` — the legacy single-prompt invocation (used when
+  the operator wants the LLM to drive all six phases inline rather
+  than via the per-phase orchestrator).
+- `iteration.md` — the iteration-strategy prompt (gap, unfiltered,
+  parity, adversarial — see `bin/run_playbook.py::next_strategy`).
+
+## Substitution conventions
+
+Most files are pure-literal markdown — the loader returns them
+unchanged. Three files use `str.format()` substitution with named
+placeholders:
+
+- `phase1.md` — `{seed_instruction}` (skip Phase 0/0b prelude when
+  `--no-seeds`) and `{role_taxonomy}` (rendered from
+  `bin.role_map.ROLE_DESCRIPTIONS`).
+- `single_pass.md` — `{skill_fallback_guide}` and
+  `{seed_instruction}`.
+- `iteration.md` — `{skill_fallback_guide}` and `{strategy}`.
+
+Inside files that go through `.format()`, JSON braces and other
+literal `{` / `}` characters MUST be doubled (`{{` / `}}`) per
+Python's format-string escaping rules. Pure-literal files do not
+need any escaping.
+
+## Editing discipline
+
+When you change a phase prompt, the loader picks up the new content
+at the next invocation — there is no caching layer to invalidate. The
+test suite at `bin/tests/test_phase_prompts_externalized.py` pins the
+loader's contract; if you add a new substitution variable, extend
+those tests.