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/plan.prompt.md

@@ -0,0 +1,52 @@
+---
+agent: agent
+description: Create a phased implementation plan before writing any code
+---
+
+# Implementation Planner
+
+Before writing any code for this feature/task, produce a structured plan.
+
+## Steps
+
+1. **Clarify the goal** — restate the requirement in one sentence; flag any ambiguities.
+2. **Research first** — identify existing utilities, libraries, or patterns in the codebase that can be reused. Do not reinvent what already exists.
+3. **Identify dependencies** — list external packages, APIs, environment variables, or database changes needed.
+4. **Break into phases** — structure work as ordered phases, each independently shippable:
+   - Phase 1: Core data model / schema changes
+   - Phase 2: Business logic + unit tests
+   - Phase 3: API / integration layer + integration tests
+   - Phase 4: UI / consumer layer + E2E tests
+5. **Identify risks** — note anything that could block progress or cause regressions.
+6. **Define done** — list the exact acceptance criteria (tests passing, coverage ≥ 80%, no lint errors, docs updated).
+
+## Output Format
+
+```
+## Goal
+[One-sentence summary]
+
+## Reuse Opportunities
+- [Existing utility/pattern]
+
+## Dependencies
+- [Package / API / env var]
+
+## Phases
+### Phase 1 — [Name]
+- [ ] Task A
+- [ ] Task B
+
+### Phase 2 — [Name]
+...
+
+## Risks
+- [Risk and mitigation]
+
+## Definition of Done
+- [ ] All tests pass (≥80% coverage)
+- [ ] No new lint errors
+- [ ] Docs updated if public API changed
+```
+
+Apply ECC coding standards throughout: immutable patterns, small focused files, explicit error handling.