feat: implement issue 0018 phase 2 — write-docs skill (first factory output)

Produces write-docs via the write-skill factory. First skill authored
end-to-end by the factory (SKILL.md via write-skill, eval via write-eval).

- Add .agents/skills/write-docs/SKILL.md (category: implement)
- Add .agents/evals/implement/write-docs/eval.yaml (6 trigger + 3 output tests)
- Remove write-a-skill placeholder (superseded by write-skill)
- Extend issue 0018 with Phase 3 (doc convention, grill-first)
- Tighten workflow steps 5a and 5d based on session lessons
- Update factory-integration-decisions: write-a-skill → write-skill
- Add 2 LESSONS.md entries (trigger gate, write-eval sub-agent bypass)
- Update docs/spec/overview.md and ROADMAP

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/notes/factory-integration-decisions.md

@@ -42,7 +42,7 @@ Categories are expressed via `metadata: category:` in SKILL.md frontmatter. Defi
 | `iac` | write-ansible-role, write-terraform-module, write-k8s-manifest, write-docker-compose, proxmox-vm-spec, iac-security-review, write-molecule-test — **global optional** |
 | `gitea` | setup-gitea-mcp, post-pr-review, create-issue — **global optional** |
 | `cross-cutting` | zoom-out, caveman, session-handoff, governance-check, git-guardrails, git-commit-message |
-| `factory` | write-a-skill, write-adr, write-workflow, write-eval, validate-skill, upgrade-skill, write-issue-spec |
+| `factory` | write-skill, write-adr, write-workflow, write-eval, validate-skill, upgrade-skill, write-issue-spec |
 | `roles` | architect, developer, reviewer, security, qa, ops — **Chunk 5** |
 
 ### 3. SKILL.md authoring standard (full)