feat(a2a): add Agent Card generation for A2A protocol discovery

A2A_IMPLEMENTATION.md

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+# A2A Protocol Support - Implementation Summary
+
+## Overview
+
+This PR implements Agent Card generation for the A2A (Agent-to-Agent) protocol, enabling Hermes Agent to be discovered by other A2A-compliant agents.
+
+**Related Issue:** https://github.com/NousResearch/hermes-agent/issues/514
+
+## What was implemented
+
+### 1. Agent Card Generation Module (`agent/a2a/`)
+
+- `agent/a2a/__init__.py` - Package entry point
+- `agent/a2a/agent_card.py` - Complete Agent Card generation with:
+  - A2A-compliant data classes (AgentCard, AgentSkill, AgentCapabilities, AgentInterface)
+  - Mapping of Hermes tools to A2A skills
+  - Configurable base URL and capabilities
+  - JSON serialization support
+
+### 2. API Server Endpoint (`gateway/platforms/api_server.py`)
+
+- Added `GET /.well-known/agent.json` endpoint
+- Returns A2A-compliant Agent Card JSON
+- Includes CORS headers for cross-origin access
+- Cache-Control headers (1 hour) to reduce server load
+
+### 3. Tests (`tests/gateway/test_api_server.py`)
+
+- `TestAgentCard` test class with 4 tests:
+  - Agent Card endpoint returns valid JSON
+  - CORS headers for cross-origin access
+  - Cache-Control headers present
+  - Skills list includes core Hermes capabilities
+
+### 4. Dependencies (`pyproject.toml`)
+
+- Added `[a2a]` optional dependency: `a2a-sdk>=1.0.0,<2`
+- Added `a2a` to `[all]` extra
+
+## Agent Card Output
+
+```json
+{
+  "name": "Hermes Agent",
+  "description": "Self-improving AI agent with memory, skills, and tool ecosystem...",
+  "version": "1.0.0",
+  "url": "http://localhost:8642/.well-known/agent.json",
+  "capabilities": {
+    "streaming": true,
+    "push_notifications": false
+  },
+  "supported_interfaces": [{
+    "url": "http://localhost:8642",
+    "protocol_binding": "JSONRPC",
+    "protocol_version": "1.0"
+  }],
+  "skills": [
+    {"id": "hermes_memory", "name": "Persistent Memory", ...},
+    {"id": "delegate_task", "name": "Task Delegation", ...},
+    ...
+  ],
+  "provider": {
+    "name": "Nous Research",
+    "url": "https://hermes-agent.nousresearch.com"
+  }
+}
+```
+
+## Testing
+
+```bash
+# Run agent card tests
+pytest tests/gateway/test_api_server.py::TestAgentCard -v
+
+# Generate agent card directly
+python -c "from agent.a2a import generate_agent_card; print(generate_agent_card().to_json())"
+```
+
+## Next Steps (Future PRs)
+
+### Phase 2: A2A Client
+- Implement `tools/a2a_tool.py` for discovering and calling remote A2A agents
+- Agent Card resolution from remote URLs
+- Task management (send message, get task, list tasks)
+
+### Phase 3: A2A Server (Full)
+- Implement JSON-RPC endpoints for A2A protocol operations
+- Task lifecycle management (create, update, complete, cancel)
+- Streaming support via Server-Sent Events
+- Push notification support
+
+## References
+
+- A2A Protocol Specification: https://a2a-protocol.org/latest/specification/
+- A2A Python SDK: https://github.com/a2aproject/a2a-python
+- Hermes Issue #514: https://github.com/NousResearch/hermes-agent/issues/514

agent/a2a/__init__.py

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+"""
+A2A (Agent-to-Agent) Protocol Support for Hermes Agent.
+
+This module provides A2A protocol capabilities:
+- Agent Card generation (/.well-known/agent.json)
+- A2A client functionality (future)
+- A2A server functionality (future)
+
+A2A is an open standard (Apache 2.0, Linux Foundation) for inter-agent communication.
+It complements MCP: MCP connects agents to tools, A2A connects agents to agents.
+
+Related: https://github.com/NousResearch/hermes-agent/issues/514
+"""
+
+from agent.a2a.agent_card import (
+    generate_agent_card,
+    get_agent_card_json,
+    get_well_known_agent_card_endpoint,
+)
+
+__all__ = [
+    "generate_agent_card",
+    "get_agent_card_json",
+    "get_well_known_agent_card_endpoint",
+]