chore(deps): remove the temporary pi-server compatibility dependency

[tt-a1i]

Problem

PR #269 is blocked by upstream earendil-works/pi#9140. @earendil-works/pi-coding-agent@0.85.0 imports @earendil-works/pi-server during startup but does not declare it, so a fresh installation of the packed OpenPI Web CLI fails.

OpenPI temporarily declares @earendil-works/pi-server@0.85.0 as a direct runtime dependency. This is a downstream compatibility shim, not an ownership change or a decision to upgrade OpenPI's supported Pi baseline.

Removal gate

Remove the direct dependency and its package-contract assertion only after all of the following are true:

• an upstream pi-coding-agent release declares pi-server, or removes/lazily isolates the startup import;
• a fresh npm installation of packed OpenPI passes the standalone Web CLI smoke test without OpenPI declaring pi-server;
• the Node 22 and Node 24 CI jobs pass at the exact OpenPI head.

Follow-up

• verify the published npm metadata, not only the upstream source;
• remove the direct dependency, lockfile entry, and regression assertion;
• link the upstream release and PR feat(ai-providers): add Antigravity and Cursor OAuth providers #269 when closing this issue.

Related: #269, #328, earendil-works/pi#9140