fix(connectors): harden Yuque stdio spec and sync dashboard type

[Yesno1314]

Follow-up to #1936 (already merged). Three hardening fixes that were not in the merged revision:

• Sync dashboard type: add stdio to mcp_mode so
  pm run build (tsc) passes with the new built-in stdio connector.
• _build_stdio_spec: append /api/v2 to the Yuque base URL when missing, so a bare site root no longer sends requests to the marketing homepage.
• _redact_mcp_configs_for_log: redact secret-like keys inside env (e.g. YUQUE_PERSONAL_TOKEN) so tokens never leak into logs.
• Add 4 unit tests covering the stdio spec, URL normalization, and env redaction.