refactor(bin): move non-spec root files into references/ across four skills

grill-with-docs, improve-codebase-architecture, tdd, and triage kept
non-spec markdown files at their skill root, in violation of
skill-audit's file-structure.md rule (only SKILL.md/README.md belong
at the root; everything else lives in scripts/, references/, assets/
or tests/). A root-level file is invisible to the ADR-0020
dangling-reference gate, which only resolves unqualified
`references/...` pointers.

- Moved and renamed to lowercase-kebab-case under references/:
  grill-with-docs (ADR-FORMAT.md, CONTEXT-FORMAT.md),
  improve-codebase-architecture (DEEPENING.md, INTERFACE-DESIGN.md,
  LANGUAGE.md), tdd (five files, casing was already fine), triage
  (AGENT-BRIEF.md, OUT-OF-SCOPE.md).
- Updated every in-skill link to the new references/ paths, including
  link text that still showed the old uppercase filenames.
- Fixed improve-codebase-architecture/SKILL.md's cross-skill citation
  of grill-with-docs's two files to the sanctioned possessive form
  with the references/ segment included.
- Updated all four skills' README.md file tables to match.
- Regenerated the flat content mirror via
  scripts/sync-plugin-content.sh --all.

Fixes #122.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PDj6F7SPXzh3FtPN78dZ88

plugins/bin/skills/tdd/references/mocking.md

@@ -0,0 +1,59 @@
+# When to Mock
+
+Mock at **system boundaries** only:
+
+- External APIs (payment, email, etc.)
+- Databases (sometimes - prefer test DB)
+- Time/randomness
+- File system (sometimes)
+
+Don't mock:
+
+- Your own classes/modules
+- Internal collaborators
+- Anything you control
+
+## Designing for Mockability
+
+At system boundaries, design interfaces that are easy to mock:
+
+**1. Use dependency injection**
+
+Pass external dependencies in rather than creating them internally:
+
+```typescript
+// Easy to mock
+function processPayment(order, paymentClient) {
+  return paymentClient.charge(order.total);
+}
+
+// Hard to mock
+function processPayment(order) {
+  const client = new StripeClient(process.env.STRIPE_KEY);
+  return client.charge(order.total);
+}
+```
+
+**2. Prefer SDK-style interfaces over generic fetchers**
+
+Create specific functions for each external operation instead of one generic function with conditional logic:
+
+```typescript
+// GOOD: Each function is independently mockable
+const api = {
+  getUser: (id) => fetch(`/users/${id}`),
+  getOrders: (userId) => fetch(`/users/${userId}/orders`),
+  createOrder: (data) => fetch('/orders', { method: 'POST', body: data }),
+};
+
+// BAD: Mocking requires conditional logic inside the mock
+const api = {
+  fetch: (endpoint, options) => fetch(endpoint, options),
+};
+```
+
+The SDK approach means:
+- Each mock returns one specific shape
+- No conditional logic in test setup
+- Easier to see which endpoints a test exercises
+- Type safety per endpoint