fix(inbox): use trusted signal report task creation

[adboio]

Problem

Inbox Create PR and Discuss currently submit billing-relevant Signal Report attribution through generic task creation. The backend is gaining a dedicated action that assigns this attribution after validating the report.

Changes

• Route Desktop Create PR and Discuss through the trusted Signal Report task action.
• Preserve implementation versus discussion relationships across desktop and mobile.
• Add a 404-only fallback so this client can deploy before the backend action.

Why

Signal-funded work must be distinguishable from ordinary Desktop work without trusting a client-selected task origin.

How did you test this?

• Built workspace dependencies successfully.
• Core test suite: 2,814 tests passed.
• Core, API client, and UI typechecks passed.
• Mobile typechecking remains blocked by unrelated existing errors; none referenced the changed files.

Automatic notifications

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

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit d788394.}

[posthog]

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