fix(mcp): downgrade protocolVersion to 2025-03-26

Claude Code CLI rejects 2025-06-18 and silently drops the connection;
2025-03-26 is the highest version it supports. Fixes #4.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/mcp/server_test.go

@@ -52,7 +52,7 @@ func TestInitialize(t *testing.T) {
 	var resp map[string]any
 	require.NoError(t, json.Unmarshal(rr.Body.Bytes(), &resp))
 	result := resp["result"].(map[string]any)
-	assert.Equal(t, "2025-06-18", result["protocolVersion"])
+	assert.Equal(t, mcp.ProtocolVersion, result["protocolVersion"])
 	si := result["serverInfo"].(map[string]any)
 	assert.Equal(t, "gitea-mcp", si["name"])
 }