feat(signals): add slack notification picker in inbox source settings

[Twixes]

Problem

Signals inbox items previously surfaced only in-app. Users wanting a heads-up when a new item lands and they're a suggested reviewer had no way to opt in.

Even after the initial Slack notifications work landed, picking a channel still required a detour through PostHog Web — users had to leave PostHog Code to install the Slack integration, then come back. That's too much friction for what should be a one-time setup.

Changes

Slack notifications surface under Inbox source settings — same surface as the existing "PR auto-start threshold" control.

• New SignalSlackNotificationsSettings component with three controls:
  • Slack workspace picker (shown only when more than one Slack Integration is connected; otherwise the single workspace is implied).
  • Notification channel picker, populated from /api/environments/{teamId}/integrations/{id}/channels/ — same endpoint as Insight Alerts. Includes an explicit "Off — don't notify me" option.
  • Minimum priority filter (P0 highest → P4, plus "every priority").
• useSlackChannels hook caches the channel list for 5 min (server-side cache is already 1 h, so cheap refreshes).
• useSignalSourceManager.handleUpdateSlackNotifications translates camelCase patches to the snake_case API body and only sends the fields the caller touched, so editing notifications doesn't wipe the autostart-priority setting (and vice versa).
• SignalUserAutonomyConfig shared type and posthogClient.updateSignalUserAutonomyConfig extended with the new optional fields.

In-app Slack connect flow so users no longer have to leave PostHog Code for PostHog Web.

• New SlackIntegrationService in the main process. Mirrors the existing GitHubIntegrationService but registers a separate slack-integration deep-link key so each provider's handler stays isolated.
• New slackIntegration tRPC router (startFlow, onCallback, onFlowTimedOut, consumePendingCallback).
• New renderer hooks: useSlackIntegrationCallback (deep-link subscriber, drains pending callbacks on mount) and useSlackConnect (button state machine — idle → connecting → idle/error/timed-out).
• The empty state in SignalSlackNotificationsSettings now shows a Connect Slack workspace button that:
  1. Asks the main process to open the browser at PostHog Cloud's /api/environments/{projectId}/integrations/authorize/?kind=slack&next=/account-connected/slack-integration?...,
  2. Waits for PostHog Cloud to complete OAuth and create the team-level Slack Integration,
  3. PostHog Cloud redirects to /account-connected/slack-integration (see the paired backend PR), which fires a posthog-code://slack-integration?integration_id=… deep link back into the app,
  4. The deep-link handler invalidates integration queries — the channel picker appears without a refresh.
• A "Connect another Slack workspace" affordance is also shown in the connected state so users with one workspace can add another without leaving the app.
• Window-focus fallback: if the deep link gets eaten by the OS/browser, returning focus to PostHog Code re-runs the integrations query.

Pairs with the backend changes in PostHog/posthog#58774.

How did you test this?

I'm an agent. Verified:

• pnpm typecheck (under apps/code) is clean — no new errors introduced.
• pnpm exec biome check is clean on every modified and new file.
• Did not start the Electron app locally; CI is authoritative for visual regressions and end-to-end behavior.

Publish to changelog?

no

---

Created with PostHog Code