feat(notifications): native OS notifications for @-mentions

[adboio]

Problem

When a teammate @-mentions you in a task thread, nothing surfaces outside the Activity feed — and there, one-row-per-task projection means the mention row is often overwritten by the next agent event before it's seen. Agent events (completed, needs input) already fire native desktop notifications; human mentions, the highest-signal event in a thread, do not.

Why

Raised while dogfooding channels: mentions from teammates were effectively invisible unless you happened to be watching the Activity page at the right moment.

Changes

• New MentionNotificationsContribution watches the task-mentions query the channels UI already polls (useMentionActivity) and dispatches new mentions through the existing NotificationBus, so mentions get the same tiering (suppressed while viewing the task / toast while focused / native + sound otherwise), notification settings gating, and click-through-to-task as agent activity. No new fetch path — one poll, one source of truth; notifications live where the spaces layout lives.
• Pure watch-state logic (baselineMentionWatch / advanceMentionWatch) lives in @posthog/core/canvas/mentionNotifications: the first page after boot or login baselines silently (the backlog isn't news), updates dedupe by message id, and a batch of several new mentions collapses into a single notification — one banner, one sound (same-task batches keep click-through to the task; mixed-task batches fall back to a count), and the auth-scoped query's removal on logout resets the baseline so another account's backlog stays silent.

How did you test this?

• Unit tests for the core watch-state (baseline, ordering, dedupe, watermark, cap) and the contribution (silent first page, per-mention dispatch shape, no re-notify on refresh, burst summary, logout re-baseline, auth-scoped meta gating) — 14 tests, all passing.
• Ran the existing notifications and canvas suites — passing.
• Biome clean; biome lint packages/core shows zero noRestrictedImports.

Automatic notifications

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

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Created with PostHog Code

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 61d5db0.}

[adboio]

CI note: the visual-regression failure here is not this PR — all 187 story screenshots pass. The job dies in "Install Visual Review CLI" because products/visual_review/cli's lockfile in PostHog/posthog went out of sync with the sharp bump (PostHog/posthog#74334, ~21:46 UTC); every PostHog/code branch run since ~21:48 fails the same way. Fix: PostHog/posthog#75785. Re-run this job once that merges.

[charlesvien]

Hey @adboio, we've migrated this repo into the PostHog/posthog monorepo, so this PR needs to be recreated there.

Easiest path: check out the monorepo and run the /porting-code-prs skill with your agent pointed at this PR. It has the migration guide and path remappings baked in so the agent doesn't have to figure them out. You don't strictly need it though, just asking your agent to port the PR works fine too.

Shout if you get stuck.