feat: implement MCP Streamable HTTP protocol endpoints

- Add POST /mcp endpoint using StreamableHTTPServerTransport
- Add HEAD /mcp endpoint returning protocol version header
- Remove old custom REST endpoints (/tools/list, /tools/call)
- Integrate marketplace tools via MCP Server decorators
- Add comprehensive MCP endpoint tests
- Update README with correct MCP protocol usage

The implementation properly handles:
- JSON-RPC 2.0 message format
- SSE (Server-Sent Events) responses
- Protocol version negotiation
- Tool filtering integration
- Authentication middleware

Tests verify:
- HEAD /mcp returns correct headers
- POST /mcp handles initialize requests
- Proper error handling for missing headers

Closes #24

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -159,30 +159,55 @@ GET /ping
 Response: {"status": "healthy"}
 ```
 
-#### List Tools
-```bash
-POST /tools/list
+#### MCP Protocol Endpoint
 
-Response: {
-  "tools": [
-    {"name": "list_issues", "description": "...", "inputSchema": {...}},
-    ...
-  ]
-}
-```
+The server implements the MCP Streamable HTTP protocol:
 
-#### Call Tool
 ```bash
-POST /tools/call
+# Check protocol version
+HEAD /mcp
+
+# Send MCP JSON-RPC requests
+POST /mcp
 Content-Type: application/json
+Accept: application/json, text/event-stream
 Authorization: Bearer YOUR_TOKEN  # If auth enabled
 
+# Example: Initialize
 {
-  "name": "list_issues",
-  "arguments": {
-    "owner": "myorg",
-    "repo": "myrepo",
-    "state": "open"
+  "jsonrpc": "2.0",
+  "id": 1,
+  "method": "initialize",
+  "params": {
+    "protocolVersion": "2024-11-05",
+    "capabilities": {},
+    "clientInfo": {
+      "name": "my-client",
+      "version": "1.0.0"
+    }
+  }
+}
+
+# Example: List tools
+{
+  "jsonrpc": "2.0",
+  "id": 2,
+  "method": "tools/list",
+  "params": {}
+}
+
+# Example: Call tool
+{
+  "jsonrpc": "2.0",
+  "id": 3,
+  "method": "tools/call",
+  "params": {
+    "name": "list_issues",
+    "arguments": {
+      "owner": "myorg",
+      "repo": "myrepo",
+      "state": "open"
+    }
   }
 }
 ```
@@ -330,7 +355,7 @@ If tool filtering is not applied:
 1. Check `.env` file syntax (no spaces around `=`)
 2. Verify comma-separated list format
 3. Check server logs for filter configuration
-4. Query `POST /tools/list` to see filtered tools
+4. Send `tools/list` MCP request to see filtered tools
 
 ## Security Considerations