feat: implement issue 0017 — factory/write-eval bootstrap skill

Adds write-eval, the first factory meta-skill. Produces eval.yaml test
files for skills following the two-section schema (trigger_tests +
output_tests) with provider-agnostic string assertions and show-plan-
then-merge-on-rerun behaviour. Hand-written bootstrap — subsequent
skills will use write-eval to produce their own evals.

Also tightens skill-implementation-workflow.md step 5b: per-section
options walk-through is now a named gate before writing, separate from
the synthesis grill. LESSONS.md entry added.

HITL behavioral test pending.

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

docs/spec/overview.md

@@ -2,12 +2,14 @@
 
 Current deployed state of this repo — what you get if you run `install.sh` today. Updated at the close of each chunk and in the same PR as any behavior change.
 
-*Last updated: 2026-05-17 (issue 0015)*
+*Last updated: 2026-05-17 (issue 0017)*
 
 ## What is deployed
 
 ### Skills
-12 skills deployed to `~/.agents/skills/` via `install.sh`. Available as slash commands in Claude Code via `~/.claude/skills/ → ~/.agents/skills/` symlink. All 12 are first-draft placeholders pending rebuild in Chunk 3.
+13 skills deployed to `~/.agents/skills/` via `install.sh`. Available as slash commands in Claude Code via `~/.claude/skills/ → ~/.agents/skills/` symlink. 12 are first-draft placeholders pending rebuild in Chunk 3; 1 is a new Chunk 3 factory skill (`write-eval`).
+
+**Factory bootstrap (Chunk 3):** `write-eval` deployed — produces `eval.yaml` test files for skills. Hand-written (bootstrap skill). Eval at `.agents/evals/factory/write-eval/eval.yaml`.
 
 Current skills: `caveman`, `diagnose`, `grill-me`, `grill-with-docs`, `improve-codebase-architecture`, `prototype`, `tdd`, `to-issues`, `to-prd`, `triage`, `write-a-skill`, `zoom-out`.
 
@@ -42,6 +44,7 @@ For chunk planning and open questions, see `docs/ROADMAP.md`.
 
 ## Recent changes
 
+- 2026-05-17 — Issue 0017 complete: `write-eval` bootstrap skill written and deployed. Two sections schema (`trigger_tests` + `output_tests`), provider-agnostic string assertions, show-plan-then-merge-on-rerun behaviour, conflict flagging (B model). Sources: agentskills/agentskills, darkrishabh/agent-skills-eval, bmad-code-org/BMAD-METHOD, mattpocock/skills. Hand-written eval at `.agents/evals/factory/write-eval/eval.yaml`.
 - 2026-05-17 — Issue 0016 complete: skill implementation workflow grill completed. `docs/notes/skill-implementation-workflow.md` written. All issues 0017–0028 updated with specific acceptance criteria. Key conventions: sub-agents prescribed at each research/writing step; conflict check against constitution + factory principles before synthesis grill; `when:` and `references:` fields added to authoring standard; write-docs moved to issue 0018 phase 2 (first factory-authored skill).
 - 2026-05-17 — Issue 0015 complete: AGENTS.md refactor implemented. Two AGENTS.md files created (`AGENTS.md` at repo root, `core/AGENTS.md` deployed to `~/.agents/AGENTS.md`). Both CLAUDE.md files slimmed to thin adapters. `deploy-manifest.sh` updated. `docs/spec/architecture.md` updated with new structure. ADR-0012 in effect.
 - 2026-05-17 — Chunk 3 issues created (0015–0028): AGENTS.md refactor prerequisite, skill workflow grill, bootstrap skills (write-eval, write-skill), factory/design/implement/test/review/deploy/operate/IaC/cross-cutting skill groups, chunk closure; all HITL; acceptance criteria for 0017–0028 to be refined after issue 0016 grill session