fix: expose project agentId through the managed-agents proxy

[devin-ai-integration]

Summary

publicProject in the managed-agents proxy dropped agentId — the canonical primary-agent id the CLI reads from project responses. Since the binding fix (4259909), opencomputer link --project and link --create-project resolve project.agentId through this endpoint; with the field stripped, binding a checkout to any project fails with Project <name> has no primary agent to bind. Fresh projects hit this unconditionally: the project exists (create succeeded) but can never be bound.

Repro: opencomputer login → opencomputer link --create-project foo → CLI creates the project, then fails at bindingFor because agentId is undefined.

The proxy already reads agentId from the upstream project to synthesize the agents list — it just never returned it. This adds ...(agentId ? { agentId } : {}) to the public shape and updates the project-proxy test to expect the field.

Verified end-to-end: deployed to a dev api-edge, link --create-project → deploy → session create now completes on a fresh example.

Link to Devin session: https://app.devin.ai/sessions/1fba978da01b46b0b064847b00455303
Open in Devin Desktop: https://app.devin.ai/desktop/session/1fba978da01b46b0b064847b00455303?variant=devin
Requested by: @motatoes

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[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

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[devin-ai-integration]

Closing — this fix was landed independently on main as 261cb816 ("Preserve project primary agent in public API", same ...(agentId ? { agentId } : {}) in publicProject + the same test assertion). After merging main, this branch's diff is empty; nothing left to ship.