docs(do-task): add DO-TASK.md + README updates (M6)

docs/DO-TASK.md covers:
- Purpose, Requirements (variant-specific prereqs + dependency-
  missing messages per variant), Reviewer CLI Requirements table
  (4 CLIs including opencode with fresh-call default).
- Install (4 subsections: Codex, Claude Code, OpenCode, Cursor).
- Per-variant Verify Installation subsections checking CLI binary,
  SKILL.md, run-review.sh, notify-telegram.sh, Superpowers
  sub-skills, and variant extras (Codex symlink, Cursor jq,
  OpenCode Superpowers ls, Cursor repo-vs-global lookup).
- Key Behavior, Dual Review Loops, Subroutine Steps, Reviewer Output
  Contract, Runtime Artifacts, Persistent Artifact Status enum
  (10 values), Failure Handling.
- Secret Scan (subroutine step 1a; per-payload; no caching) with
  canonical 10-pattern regex list and redaction contract.
- Supported Reviewer CLIs table (4 rows, including opencode).
- Notifications, Template Guardrails (14 core sections + Runtime
  State keys), Variant Hardening Notes (4 subsections), Execution
  Workflow Rules.

docs/README.md adds DO-TASK.md entry.

README.md:
- Skills table adds 4 do-task rows (codex, claude-code, opencode,
  cursor).
- Docs links add "Do-task guide" entry.
- Repository Layout adds do-task/ subdirectory.

Reviewer: codex / gpt-5.4. Approved round 2:
- Round 1: 2 P2 (prereqs inaccurate, Verify Installation incomplete)
  + 1 P3 -> REVISE.
- Round 2: 0 P0/P1/P2/P3 -> APPROVED.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/README.md

@@ -6,6 +6,7 @@ This directory contains user-facing docs for each skill.
 
 - [ATLASSIAN.md](./ATLASSIAN.md) — Includes requirements, generated bundle sync, install, auth, safety rules, and usage examples for the Atlassian skill.
 - [CREATE-PLAN.md](./CREATE-PLAN.md) — Includes requirements, install, verification, and execution workflow for create-plan.
+- [DO-TASK.md](./DO-TASK.md) — Single-prompt end-to-end execution with dual reviewer loops (plan + implementation), TDD-first, single task commit. Sibling of create-plan/implement-plan scoped to ad-hoc tasks.
 - [IMPLEMENT-PLAN.md](./IMPLEMENT-PLAN.md) — Includes requirements, install, verification, and milestone review workflow for implement-plan.
 - [TELEGRAM-NOTIFICATIONS.md](./TELEGRAM-NOTIFICATIONS.md) — Shared Telegram notification setup used by reviewer-driven skills.
 - [WEB-AUTOMATION.md](./WEB-AUTOMATION.md) — Includes requirements, install, dependency verification, and usage examples for web-automation.