chore: bootstrap repo with chunks 1 and 2

Establishes the global AI development config repo from scratch:

- Chunk 1: repo skeleton, install.sh, statusline, deploy manifest
- Chunk 2: core instructions (coding/git/testing), CLAUDE.md rewrite
  (always-on + content index two-tier model), docs restructure,
  6 ADRs, ROADMAP.md, .gitkeep placeholders
- Bootstrap skills in .claude/skills/ (to be catalogued and migrated
  to .agents/skills/ in Chunk 3)

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

.claude/skills/tdd/interface-design.md

@@ -0,0 +1,31 @@
+# Interface Design for Testability
+
+Good interfaces make testing natural:
+
+1. **Accept dependencies, don't create them**
+
+   ```typescript
+   // Testable
+   function processOrder(order, paymentGateway) {}
+
+   // Hard to test
+   function processOrder(order) {
+     const gateway = new StripeGateway();
+   }
+   ```
+
+2. **Return results, don't produce side effects**
+
+   ```typescript
+   // Testable
+   function calculateDiscount(cart): Discount {}
+
+   // Hard to test
+   function applyDiscount(cart): void {
+     cart.total -= discount;
+   }
+   ```
+
+3. **Small surface area**
+   - Fewer methods = fewer tests needed
+   - Fewer params = simpler test setup