feat: implement ADR-0012 — AGENTS.md refactor (issue 0015)

Create AGENTS.md at repo root and core/AGENTS.md as provider-agnostic
sources of always-on rules. Slim both CLAUDE.md files to thin adapters.
Deploy core/AGENTS.md → ~/.agents/AGENTS.md via deploy-manifest.sh.
Update three test suites to check correct locations post-refactor.
All automated tests pass (57 instructions, 106 install, 25 governance).
HITL behavioral test plan in test-instructions-and-docs.sh scenarios 9–12.

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

docs/spec/overview.md

@@ -2,7 +2,7 @@
 
 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*
+*Last updated: 2026-05-17 (issue 0015)*
 
 ## What is deployed
 
@@ -16,7 +16,8 @@ Current skills: `caveman`, `diagnose`, `grill-me`, `grill-with-docs`, `improve-c
 **Prerequisite:** AGENTS.md refactor must complete before Chunk 3 skill implementation begins — both `CLAUDE.md` files (repo-level and global) become thin adapters importing `AGENTS.md`. See ADR-0012 (to be written during implementation).
 
 ### Claude Code configuration
-- `~/.claude/CLAUDE.md` — global config index; always-on rules + content index pointers
+- `~/.claude/CLAUDE.md` — thin adapter; imports `~/.agents/AGENTS.md` (Communication + Behavior) and `governance.md`; content index pointers only
+- `~/.agents/AGENTS.md` — global always-on rules (Communication + Behavior); provider-agnostic source of truth
 - `~/.claude/core/instructions/` — coding, git, testing, governance instruction files
 - `~/.claude/settings.json` — Claude Code settings
 
@@ -41,6 +42,7 @@ For chunk planning and open questions, see `docs/ROADMAP.md`.
 
 ## Recent changes
 
+- 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
 - 2026-05-17 — behavioral tests fully resolved: `CONTEXT.md` now always-loaded via `@import` in repo `CLAUDE.md`; standing rule added to check `docs/adr/` and ROADMAP resolved entries before answering design questions; communication/behavior and secrets rules tightened; Chunk 2 and Governance Phase 1 ✅ complete
 - 2026-05-17 — added `LESSONS.md` (issue 0013) and `docs/spec/` (issue 0014); refactored `docs/VISION.md` to goals/intent only