feat: add GitHub Copilot prompt support

Adds GitHub Copilot VS Code instruction and prompt files for ECC workflows, with VS Code prompt frontmatter/settings aligned to current docs and tests covering the surface.

Co-authored-by: Girish Kanjiyani <girish.kanjiyani5040@gmail.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/prompts/tdd.prompt.md

@@ -0,0 +1,47 @@
+---
+agent: agent
+description: Test-driven development cycle — write the test first, then implement
+---
+
+# TDD Workflow
+
+Follow the RED → GREEN → IMPROVE cycle strictly. Do not write implementation code before a failing test exists.
+
+## Cycle
+
+### 1. RED — Write the failing test
+- Write a test that describes the desired behavior.
+- Run it. It **must fail** before continuing.
+- Use Arrange-Act-Assert structure.
+- Name tests descriptively: `returns empty array when no items match filter`, not `test itemFilter`.
+
+### 2. GREEN — Minimal implementation
+- Write the **minimum** code needed to make the test pass.
+- Do not over-engineer at this stage.
+- Run the test again — it **must pass**.
+
+### 3. IMPROVE — Refactor
+- Clean up duplication, naming, structure.
+- Keep all tests passing after each change.
+- Check coverage: target **≥ 80%**.
+
+## Test Layer Checklist
+
+- [ ] **Unit** — pure functions, utilities, isolated components
+- [ ] **Integration** — API endpoints, database operations, service boundaries
+- [ ] **E2E** — at least one critical user flow covered
+
+## Quality Gates
+
+Before marking the feature done:
+- [ ] All tests pass
+- [ ] Coverage ≥ 80%
+- [ ] No skipped/commented-out tests
+- [ ] Edge cases covered: empty input, nulls, boundary values, error paths
+
+## Anti-patterns to Avoid
+
+- Writing implementation before tests
+- Testing implementation details instead of behavior
+- Mocking too deeply (prefer integration tests over excessive mocks)
+- Assertions that always pass (`expect(true).toBe(true)`)