Add external data source signal sources (GitHub, Linear, Zendesk)

[Twixes]

Problem

Signal sources only supported session replay and LLM analytics. We want users to also pull in GitHub issues, Linear issues, and Zendesk tickets as signal sources - but these require connecting an external data source first (OAuth for GitHub/Linear, API key for Zendesk).

Changes

Changes, as described by Claude:

• Use /api/projects/... consistently for external data source schema updates (updateExternalDataSchema)
• Add new OAuth scopes for external data sources and bump scope version
• Build DataSourceSetup component with per-source setup flows (GitHub repo picker, Linear OAuth + polling, Zendesk credentials form)
• Extend SignalSourceToggles to show a "Connect" button for sources that need setup before they can be toggled
• Extract signal source management logic into useSignalSourceManager hook (replaces the old signalSourceSelectionsStore and the inline logic that was duplicated between settings and onboarding) + update SignalsStep, SignalSourcesSettings, and useTutorialTour to use the new hook

[Twixes]

• Add external data source signal sources (GitHub, Linear, Zendesk) #1262 👈 (View in Graphite)
• Add Linear integration service #1261
• Simplify GitHub integration flow #1260
• main

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[Twixes]

Merge activity

• Mar 17, 11:47 AM UTC: A user started a stack merge that includes this pull request via Graphite.
• Mar 17, 11:49 AM UTC: Graphite rebased this pull request as part of a merge.
• Mar 17, 11:54 AM UTC: @Twixes merged this pull request with Graphite.