feat(agent): add isolated cloud computer use

[richardsolomou]

Problem

Cloud task agents need a computer they can control without reaching back into the user's Mac. Local computer control and cloud-owned computer control have different trust and lifecycle boundaries.

Changes

• Send an opt-in computer-use capability when creating cloud runs and bypass pre-warmed runs that lack it.
• Expose the shared computer tools to cloud Linux sessions while keeping local control limited to macOS sessions.
• Implement Linux screenshot, application, mouse, and keyboard actions against the task sandbox display.
• Remove the computer-specific desktop relay behavior while preserving the generic cloud MCP relay.

Companion sandbox and backend changes: PostHog/posthog#72186

How did you test this?

• Focused Vitest suites: 24 agent tests, 68 API client tests, and 32 core task-creation tests passed.
• pnpm build:deps, including the packaged local-tools MCP smoke check.
• pnpm typecheck across the full workspace.
• Biome checks on changed files. A broader package lint reports existing warnings outside this diff.
• I could not exercise the real virtual desktop from this environment because the companion sandbox image requires Docker.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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Created with PostHog Code

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 98441cb.}

[github-actions]

This PR has had no activity for 7 days and has been marked stale. We are moving to the monorepo and tightening PR staleness in preparation, so it will be closed in 7 days if no further activity occurs.

[charlesvien]

Closing stale PRs ahead of Friday's monorepo migration. If this is a mistake, rebase and reopen for review.