fix(canvas): render full activity messages and native references

[puemos]

Problem

Structured references in task comments were shown as raw XML-like tags instead of native inline chips.

Changes

• Render the shared structured-reference format through existing native chip components in task comments and activity previews.
• Keep chips inline and truncate long labels without introducing new lines.
• Validate GitHub references before making them interactive.

Why: Task discussions should display references consistently with the composer and agent transcript instead of exposing their serialization format.

Show me!!

How did you test this?

• pnpm --filter @posthog/ui exec vitest run src/features/canvas/components/MentionText.test.tsx
• pnpm --filter @posthog/ui typecheck
• pnpm exec biome check packages/ui/src/features/canvas/components/MentionText.tsx packages/ui/src/features/canvas/components/MentionText.test.tsx

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 458f1ce.}

[posthog]

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