ci: add weekly feedback writer workflow to fix unhelpful docs pages

[devin-ai-integration]

Summary

Adds a weekly GitHub Actions workflow (feedback-writer.yml) that automatically finds docs pages with negative reader feedback and kicks off Fern Writer runs to improve them.

How it works:

1. Runs every Monday at 9 AM UTC (also manually triggerable via workflow_dispatch)
2. Queries PostHog HogQL API for feedback_submitted events where satisfied = false on buildwithfern.com/learn from the past 7 days
3. Groups feedback by page URL, ranks pages by number of unhelpful votes
4. For each page (up to 5 by default), constructs a prompt summarizing the complaints (reasons + reader comments) and calls the FAI Scribe API (POST /scribe/sessions) to start a Fern Writer run that opens a PR against this repo
5. Logs a summary of started/failed runs

Manual dispatch inputs:

• dry_run — preview prompts without starting runs
• days — lookback window (default 7)
• max_runs — cap on Fern Writer runs per execution (default 5)

Required secrets:

Secret	Purpose
POSTHOG_ANALYTICS_API_KEY	PostHog personal API key for HogQL queries
POSTHOG_ANALYTICS_PROJECT_ID	PostHog project ID
FAI_API_TOKEN	Bearer token for the FAI Scribe API
FAI_SERVER_URL	FAI base URL (optional, defaults to https://fai.buildwithfern.com)

Link to Devin session: https://app.devin.ai/sessions/92eb363287c2470eadc66ae88e1f199b

[devin-ai-integration]

🤖 Devin AI Engineer

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✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1781553941-feedback-writer-workflow.docs.buildwithfern.com/learn