v1.18.0

Added

• Private/custom manifest overlay: define your own provisionable MCP servers in
  ~/.pmcp/manifest.yaml (user), <project>/.pmcp/manifest.yaml (project), or a
  file pointed to by PMCP_MANIFEST_PATH, merged over the shipped manifest
  without editing it. Precedence is shipped < user < project < PMCP_MANIFEST_PATH
  (same-named entries are replaced whole). Overlay parsing is fail-soft — a
  missing file is skipped, and a malformed file or a single bad entry logs a
  warning and is skipped without crashing the gateway. Merged servers
  participate identically in gateway.request_capability, gateway.catalog_search,
  gateway.provision, and startup refresh resolution, gated by the same
  policy/auth checks.