feat: implement A2A protocol for fleet-wizard delegation (#1122)

Implements Google Agent2Agent Protocol v1.0 with full fleet integration:

## Phase 1 - Agent Card & Discovery
- Agent Card types with JSON serialization (camelCase, Part discrimination by key)
- Card generation from YAML config (~/.hermes/agent_card.yaml)
- Fleet registry with LocalFileRegistry + GiteaRegistry backends
- Discovery by skill ID or tag

## Phase 2 - Task Delegation
- Async A2A client with JSON-RPC SendMessage/GetTask/ListTasks/CancelTask
- FastAPI server with pluggable task handlers (skill-routed)
- CLI tool (bin/a2a_delegate.py) for fleet delegation
- Broadcast to multiple agents in parallel

## Phase 3 - Security & Reliability
- Bearer token + API key auth (configurable per agent)
- Retry logic (max 3 retries, 30s timeout)
- Audit logging for all inter-agent requests
- Error handling per A2A spec (-32001 to -32009 codes)

## Test Coverage
- 37 tests covering types, card building, registry, server integration
- Auth (required + success), handler routing, error handling

Files:
- nexus/a2a/ (types.py, card.py, client.py, server.py, registry.py)
- bin/a2a_delegate.py (CLI)
- config/ (agent_card.example.yaml, fleet_agents.json)
- docs/A2A_PROTOCOL.md
- tests/test_a2a.py (37 tests, all passing)

config/agent_card.example.yaml

@@ -0,0 +1,57 @@
+# A2A Agent Card Configuration
+# Copy this to ~/.hermes/agent_card.yaml and customize.
+#
+# This file drives the agent card served at /.well-known/agent-card.json
+# and used for fleet discovery.
+
+name: "timmy"
+description: "Sovereign AI agent — consciousness, perception, and reasoning"
+version: "1.0.0"
+
+# Network endpoint where this agent receives A2A tasks
+url: "http://localhost:8080/a2a/v1"
+protocol_binding: "HTTP+JSON"
+
+# Supported input/output MIME types
+default_input_modes:
+  - "text/plain"
+  - "application/json"
+
+default_output_modes:
+  - "text/plain"
+  - "application/json"
+
+# Capabilities
+streaming: false
+push_notifications: false
+
+# Skills this agent advertises
+skills:
+  - id: "reason"
+    name: "Reason and Analyze"
+    description: "Deep reasoning and analysis tasks"
+    tags: ["reasoning", "analysis", "think"]
+
+  - id: "code"
+    name: "Code Generation"
+    description: "Write, review, and debug code"
+    tags: ["code", "programming", "debug"]
+
+  - id: "research"
+    name: "Research"
+    description: "Web research and information synthesis"
+    tags: ["research", "web", "synthesis"]
+
+  - id: "memory"
+    name: "Memory Query"
+    description: "Query agent memory and past sessions"
+    tags: ["memory", "recall", "context"]
+
+# Authentication
+# Options: bearer, api_key, none
+auth:
+  scheme: "bearer"
+  token_env: "A2A_AUTH_TOKEN"       # env var containing the token
+  # scheme: "api_key"
+  # key_name: "X-API-Key"
+  # key_env: "A2A_API_KEY"