refactor(canvas): share thread and PR artifact logic (1/4)

[puemos]

Problem

Task thread behavior and pull request artifact rendering were duplicated across channel surfaces, making the upcoming Activity view harder to review and easier to drift.

Changes

• Extract shared thread conversation behavior and PR artifact presentation.
• Adopt both helpers in the existing Thread panel and channel artifacts view.
• No new layout or flag-gated experience is activated in this PR.

This is PR 1 of the channel spaces stack and is safe with the layout flag either off or on: both states retain the existing experience.

How did you test this?

• pnpm exec vitest run src/features/canvas/components/ThreadPanel.test.tsx src/utils/posthogLinks.test.ts
• pnpm typecheck in packages/ui

Automatic notifications

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

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Created with PostHog Code

[trunk-io]

😎 Merged successfully - details.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit e04e1a2.}

[puemos]

• feat(channels): add the structured Activity view (4/4) #3815
• feat(channels): add the flag-gated spaces layout (3/4) #3814
• feat(channels): add scoped channel foundations (2/4) #3813
• refactor(canvas): share thread and PR artifact logic (1/4) #3812 👈 (View in Graphite)
• main

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[puemos]

Merge activity

• Jul 28, 8:32 AM UTC: A user started a stack merge that includes this pull request via Graphite.
• Jul 28, 8:52 AM UTC: Graphite couldn't merge this PR because it failed for an unknown reason (GitHub is reporting that this PR is not mergeable, despite passing required status checks defined by your branch protection rules. Please check your rulesets for additional blocking criteria. Graphite Merge Queue does not currently support rulesets. Please contact Graphite support for further assistance.).