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/build-fix.prompt.md

@@ -0,0 +1,47 @@
+---
+agent: agent
+description: Systematically diagnose and fix build errors, type errors, or failing CI
+---
+
+# Build Error Resolution
+
+Work through the error systematically. Fix root causes — do not suppress warnings or skip checks.
+
+## Process
+
+### 1. Capture the full error
+Paste or describe the complete error output (not just the last line). Include:
+- Error message and stack trace
+- File and line number if shown
+- Build tool and command that failed
+
+### 2. Categorize the error
+
+| Category | Signals |
+|----------|---------|
+| **Type error** | `Type X is not assignable to Y`, `Property does not exist` |
+| **Import/module** | `Cannot find module`, `does not provide an export` |
+| **Syntax** | `Unexpected token`, `Expected ;` |
+| **Dependency** | `peer dep conflict`, `missing package`, `version mismatch` |
+| **Environment** | `command not found`, `ENOENT`, missing env var |
+| **Test failure** | `expected X but received Y`, assertion failure |
+| **Lint** | `ESLint`, `no-unused-vars`, `no-console` |
+
+### 3. Fix strategy
+
+- **Type errors** — fix the type, do not cast to `any` or `unknown` unless truly unavoidable.
+- **Import errors** — verify the export exists; check for circular dependencies.
+- **Dependency errors** — update lockfile, reconcile peer dep versions, do not delete `node_modules` as a first step.
+- **Test failures** — fix the implementation if behavior is wrong; fix the test only if the test itself is incorrect.
+- **Lint errors** — fix the code, do not add `// eslint-disable` unless the rule is genuinely inapplicable and you document why.
+
+### 4. Verify the fix
+After applying a fix, run the build/test command again. Confirm the specific error is resolved and no new errors were introduced.
+
+### 5. Check for related issues
+A single root cause often produces multiple error messages. After fixing, scan for similar patterns elsewhere in the codebase.
+
+## Rules
+- Never use `--no-verify` to skip hooks.
+- Never suppress type errors with `@ts-ignore` without a comment explaining why.
+- Never delete lock files without understanding why they are conflicting.