chore: add repository-specific copilot instructions

.github/copilot-instructions.md

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+# validate-a2a - GitHub Copilot Instructions
+
+## ABSOLUTE RULES - NO EXCEPTIONS
+
+### 1. ALL WORK VIA PULL REQUESTS
+- **NEVER commit directly to `main`.** All changes MUST go through PRs.
+
+### 2. LOCAL CI VALIDATION BEFORE PUSH
+- Test the action locally before pushing
+
+### 3. NO WATCH/BLOCKING COMMANDS
+- **NEVER run blocking commands** without timeout
+
+---
+
+## Repository Purpose
+
+**validate-a2a** is a GitHub Action that validates A2A protocol agent cards.
+It provides three-dimensional scoring, cryptographic verification, and live endpoint testing.
+
+Users add it to their CI pipelines to validate their agent-card.json files.
+
+**Technology Stack**: TypeScript, GitHub Actions
+
+**Current Version**: v2.4.0
+**Default Branch:** `main`
+
+## How It Works
+
+```yaml
+# Usage in a workflow
+- uses: capiscio/validate-a2a@v2
+  with:
+    agent-card: './agent-card.json'
+    strict: 'true'
+    test-live: 'false'
+```
+
+### Key Inputs
+
+| Input | Description | Default |
+|-------|-------------|---------|
+| `agent-card` | Path or URL to agent-card.json | `./agent-card.json` |
+| `strict` | Strict mode for production compliance | `false` |
+| `test-live` | Test live endpoint with A2A messages | `false` |
+| `skip-signature` | Skip cryptographic signature checks | `false` |
+
+## Structure
+
+```
+validate-a2a/
+├── action.yml               # GitHub Action definition
+├── src/                     # TypeScript source
+├── dist/                    # Compiled output (committed for Actions)
+├── package.json
+└── tsconfig.json
+```
+
+## Critical Rules
+
+- **`dist/` must be committed** — GitHub Actions run from the repo, not npm
+- After any `src/` change: `pnpm build` then commit `dist/`
+- Validation logic should use capiscio-core where possible
+- Version must stay aligned with capiscio-core
+
+## Publishing
+
+Tag push triggers the action version update:
+```bash
+git tag v2.4.1 && git push origin v2.4.1
+```
+Also update the major version tag:
+```bash
+git tag -f v2 && git push origin v2 --force   # Users reference @v2
+```