refactor(mobile): reuse the cloud task engine

[richardsolomou]

Problem

Desktop and mobile independently implemented cloud-task streaming, hydration, retry, and deduplication behavior. Keeping both implementations makes parity fixes surface-specific.

Depends on #3623
Refs #2416

Changes

I extracted the existing desktop cloud-task implementation into a host-neutral engine and kept the desktop service as a thin wrapper. Mobile supplies host adapters to the same engine. Terminal runs retain the archived S3 log fallback when persisted session logs are empty.

Follow-up: #3608

How did you test this?

• Ran the full core and mobile suites, including archived-log fallback coverage.
• Ran the monorepo typecheck.
• Ran repo-wide Biome and the host-boundary check.

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 5e38056.}

[richardsolomou]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat(mobile): add Codex task parity #3654
• refactor(mobile): finalize portability boundary #3642
• refactor(mobile): reuse inbox semantics #3641
• refactor(mobile): reuse session display semantics #3640
• refactor(core): own composer model policy #3637
• refactor(mobile): reuse transport and permission policies #3635
• refactor(mobile): reuse inbox transport #3634
• refactor(mobile): reuse repository aggregation #3633
• refactor(mobile): finish shared cloud task adoption #3608
• refactor(mobile): reuse the cloud task engine #3624 👈 (View in Graphite)
• refactor(mobile): reuse core task policies #3623
• refactor(mobile): reuse the task API client #3622
• refactor(mobile): reuse task contracts and model policy #3616
• main

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[greptile-apps]

Prompt To Fix All With AI

Fix the following 2 code review issues. Work through them one at a time, proposing concise fixes.

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### Issue 1 of 2
packages/core/src/cloud-task/cloud-task-engine.ts:2165
**Archived fetch can hang hydration**

When a terminal run has no persisted session-log rows and its archive request stalls, this unbounded fetch never emits a snapshot or retryable error, leaving the mobile task session loading indefinitely.

### Issue 2 of 2
packages/core/src/cloud-task/cloud-task-engine.ts:2169-2170
**Malformed line discards entire archive**

When an archived NDJSON log contains one malformed line, parsing the complete array throws and returns `null`, causing hydration to reject every otherwise valid entry; the previous mobile parser skipped malformed lines and retained the remaining task history.

_{Reviews (1): Last reviewed commit: "fix(core): restore archived cloud task l..." | Re-trigger Greptile}

[veria-ai]

PR overview

All previously flagged issues have been addressed. No open security concerns remain on this pull request.

Security review

No open security issues remain on this pull request.

Fixed/addressed: 1 · PR risk: 0/10