v0.7.3

Industry AI Workflows 0.7.3

  uam-mcp 0.7.3
  uat-mcp 0.7.2 (unchanged)
  upa-mcp 0.7.3

Asset Manager (`uam-mcp`) and Pipeline Automation (`upa-mcp`) move to 0.7.3.
Asset Transformer (`uat-mcp`) has no changes and stays at 0.7.2, so
`/plugin update` does not offer it.

### Changed

- **Per-plugin attribution.** Asset Manager now sends
  `X-Unity-Cloud-Api-Source: uam_mcp@<version>` and `User-Agent: UAM_MCP/...`;
  Pipeline Automation sends `upa_mcp@<version>` and `UPA_MCP/...`. Both sent
  `uap_mcp` / `UAP_MCP` through 0.7.2, which left the calls they share (token
  refresh, OIDC discovery) unattributable to either plugin. Gateway queries
  should match all three values until older installs have updated. The
  `UAP_MCP_USER_AGENT` override, `UAP_MCP_HOME` and the shared `~/.uap_mcp`
  token cache are unchanged, so nobody is signed out.

One squashed commit per release, built from the internal trunk.