Simplify GitHub integration flow

[Twixes]

Problem

The GitHub integration service had a whole callback server (HTTP in dev, deep links in prod) to detect when the user completed the OAuth flow. This was overly complex and fragile - it managed sockets, timeouts, and HTML responses just to know "did the user click authorize."

Changes

• Rip out the callback server and deep link machinery from GitHubIntegrationService - now it just opens the authorize URL in the browser and returns immediately
• Extract shared integration flow schemas into integration-flow-schemas.ts so Linear (and future integrations) can reuse them
• Update GitIntegrationStep to poll for the integration instead of waiting for a callback
• Remove the cancelFlow endpoint since there's nothing to cancel anymore

[Twixes]

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

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