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>

README.md

@@ -34,6 +34,11 @@ ai-coding-skills/
 │   │   ├── claude-code/
 │   │   ├── opencode/
 │   │   └── cursor/
+│   ├── do-task/
+│   │   ├── codex/
+│   │   ├── claude-code/
+│   │   ├── opencode/
+│   │   └── cursor/
 │   ├── implement-plan/
 │   │   ├── codex/
 │   │   ├── claude-code/
@@ -64,6 +69,10 @@ ai-coding-skills/
 | create-plan | claude-code | Structured planning with milestones, iterative cross-model review, and runbook-first execution workflow | Ready | [CREATE-PLAN](docs/CREATE-PLAN.md) |
 | create-plan | opencode | Structured planning with milestones, iterative cross-model review, and runbook-first execution workflow | Ready | [CREATE-PLAN](docs/CREATE-PLAN.md) |
 | create-plan | cursor | Structured planning with milestones, iterative cross-model review, and runbook-first execution workflow | Ready | [CREATE-PLAN](docs/CREATE-PLAN.md) |
+| do-task | codex | Single-prompt end-to-end execution with dual reviewer loops (plan + implementation), TDD-first, single task commit | Ready | [DO-TASK](docs/DO-TASK.md) |
+| do-task | claude-code | Single-prompt end-to-end execution with dual reviewer loops (plan + implementation), TDD-first, single task commit | Ready | [DO-TASK](docs/DO-TASK.md) |
+| do-task | opencode | Single-prompt end-to-end execution with dual reviewer loops (plan + implementation), TDD-first, single task commit | Ready | [DO-TASK](docs/DO-TASK.md) |
+| do-task | cursor | Single-prompt end-to-end execution with dual reviewer loops (plan + implementation), TDD-first, single task commit | Ready | [DO-TASK](docs/DO-TASK.md) |
 | implement-plan | codex | Worktree-isolated plan execution with iterative cross-model milestone review | Ready | [IMPLEMENT-PLAN](docs/IMPLEMENT-PLAN.md) |
 | implement-plan | claude-code | Worktree-isolated plan execution with iterative cross-model milestone review | Ready | [IMPLEMENT-PLAN](docs/IMPLEMENT-PLAN.md) |
 | implement-plan | opencode | Worktree-isolated plan execution with iterative cross-model milestone review | Ready | [IMPLEMENT-PLAN](docs/IMPLEMENT-PLAN.md) |
@@ -75,6 +84,7 @@ ai-coding-skills/
 - Docs index: `docs/README.md`
 - Atlassian guide: `docs/ATLASSIAN.md`
 - Create-plan guide: `docs/CREATE-PLAN.md`
+- Do-task guide: `docs/DO-TASK.md`
 - Implement-plan guide: `docs/IMPLEMENT-PLAN.md`
 - Web-automation guide: `docs/WEB-AUTOMATION.md`