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>

tests/test-install.sh

@@ -52,6 +52,14 @@ while IFS= read -r -d '' deployed; do
     fi
 done < <(find "$TEMP_HOME/.claude/core" -type f -print0)
 
+echo ""
+echo "--- core/AGENTS.md → ~/.agents/AGENTS.md (0015) ---"
+if diff -q "$REPO_ROOT/core/AGENTS.md" "$TEMP_HOME/.agents/AGENTS.md" > /dev/null 2>&1; then
+    pass "core/AGENTS.md deployed and matches source"
+else
+    fail "~/.agents/AGENTS.md — missing or differs from source"
+fi
+
 echo ""
 echo "--- skills deployed to ~/.agents/skills/ ---"
 while IFS= read -r -d '' src_skill; do
@@ -168,6 +176,13 @@ else
     fail "idempotent: ~/.claude/skills symlink broken after second install"
 fi
 
+# AGENTS.md still correct after second run
+if diff -q "$REPO_ROOT/core/AGENTS.md" "$TEMP_HOME/.agents/AGENTS.md" > /dev/null 2>&1; then
+    pass "idempotent: ~/.agents/AGENTS.md correct after second install"
+else
+    fail "idempotent: ~/.agents/AGENTS.md corrupted after second install"
+fi
+
 echo ""
 echo "Results: $PASS passed, $FAIL failed"
 [[ $FAIL -eq 0 ]]