feat: add Streamable HTTP transport support

Add support for running the MCP server over HTTP with Server-Sent Events
(SSE) using the MCP Streamable HTTP specification, alongside the existing
STDIO transport.

New features:
- Transport abstraction with Transport interface
- HTTP transport with session management
- SSE support for server-initiated notifications
- CORS security with configurable allowed origins
- Optional TLS support
- CLI flags for HTTP configuration (--transport, --http-address, etc.)
- NixOS module options for HTTP transport

The HTTP transport implements:
- POST /mcp: JSON-RPC requests with session management
- GET /mcp: SSE stream for server notifications
- DELETE /mcp: Session termination
- Origin validation (localhost-only by default)

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

internal/mcp/transport.go

@@ -0,0 +1,10 @@
+package mcp
+
+import "context"
+
+// Transport defines the interface for MCP server transports.
+type Transport interface {
+	// Run starts the transport and blocks until the context is cancelled
+	// or an error occurs.
+	Run(ctx context.Context) error
+}