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/refactor.prompt.md

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+---
+agent: agent
+description: Clean up dead code, reduce duplication, and simplify structure without changing behavior
+---
+
+# Refactor & Cleanup
+
+Improve the internal structure of the selected code without changing its observable behavior. All tests must pass before and after.
+
+## Before Starting
+- [ ] Confirm the test suite is passing.
+- [ ] Note the current coverage baseline.
+- [ ] Identify the scope: single function, file, or module?
+
+## Refactoring Targets
+
+### Dead Code Removal
+- Unused variables, imports, functions, and exports
+- Commented-out code blocks (delete, don't leave as comments)
+- Feature flags that are permanently enabled/disabled
+- Unreachable branches
+
+### Duplication Reduction
+- Repeated logic that can be extracted into a shared utility
+- Copy-pasted blocks differing only in a parameter (extract with that parameter)
+- Inline constants that appear in multiple places (extract to named constants)
+
+### Structure Improvements
+- Functions over 50 lines → break into smaller, named steps
+- Files over 800 lines → extract cohesive sub-modules
+- Nesting deeper than 4 levels → extract early-return guards or helper functions
+- Mixed concerns in one function → split into focused single-responsibility functions
+
+### Naming
+- Rename variables/functions whose names don't match their behavior
+- Replace magic numbers and strings with named constants
+- Align naming with the domain language used elsewhere in the codebase
+
+## Constraints
+- **No behavior changes** — refactoring is purely structural.
+- **One concern at a time** — do not mix refactoring with feature work or bug fixes.
+- **Keep tests green** — run the suite after each meaningful change.
+- **Don't add abstractions preemptively** — extract only what has already proven to be duplicated (rule of three).
+
+## Output
+After refactoring, summarize:
+- What was removed (dead code, duplication)
+- What was extracted (new utilities, constants)
+- What was renamed and why
+- Coverage before / after (should not decrease)