feat(signals): view agent-proposed fix as a diff

[Twixes]

Problem

When the signals research agent flags a report as actionable, we want to see its proposed fix as a diff inside PostHog Code — without spinning the sandbox back up.

Changes

• Fetch the PROPOSED_FIX_PATCH artefact for a signal report
• Unpack the JSON envelope {base_sha, base_ref, stats, patch} and render patch in the built-in diff viewer
• Stats banner (files / +lines / -lines) above the diff
• Drift indicator when base_sha no longer matches current origin/main
• Graceful empty state when the artefact isn't there yet (not actionable, or still running)

(Fill me in once it lands.)

How did you test this?

I'm an agent — nothing tested yet.

Publish to changelog?

no

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Pairs with https://github.com/PostHog/posthog PR for the producer side.

Created with PostHog Code