refactor(cloud): share task contracts and model policy

[richardsolomou]

Problem

Desktop and mobile defined overlapping task, run, automation, model, reasoning, and session contracts. Those copies could drift before either host reached shared orchestration.

Refs #2416

Changes

I made @posthog/shared the canonical contract and model-policy layer, then moved the agent, workspace server, and mobile consumers onto it in the same PR. Mobile imports shared contracts directly rather than keeping compatibility re-export files.

Follow-up: #3622

How did you test this?

• Ran 690 shared tests and 435 mobile tests.
• Built the required workspace packages and ran agent and workspace-server typechecks.
• Ran Biome and diff checks.

Automatic notifications

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

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Created with PostHog Code

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit f08bac5.}