fix(cli): resolve the basepath of a preview when deleting by --id

[devin-ai-integration]

Description

Linear ticket: Refs

fern docs preview delete --id <id> built the preview hostname from the org + ID and sent that to FDR. FDR keys a docs site on hostname + basepath, so for a preview published under a basepath (acme-preview-abc.docs.buildwithfern.com/docs) nothing was deleted — the DB rows survived, the S3 assets (deleted by hostname prefix) did not, and the CLI still printed "Successfully deleted" with exit code 0.

The ID now resolves to the URL FDR actually stores, by looking the deployment up in the same listing fern docs preview list uses (listAllDocsUrls({ preview: true }), org-scoped server-side) and keeping its basePath.

Changes Made

• resolvePreviewUrlFromId looks up the preview by hostname and returns domain + basePath; root previews behave exactly as before.

• Fails loudly instead of silently deleting the wrong thing: no match → "No preview deployment found for ID ..." pointing at fern docs preview list; a host serving several basepaths → lists them and asks for the full URL.

• Login now happens before ID resolution (the lookup needs the token).

• toPreviewUrl extracted from listDocsPreview so the list and delete paths build the URL identically.

• Updated README.md generator (if applicable)

Testing

• Unit tests added/updated — findPreviewsForHostname covering basepath preservation, root previews, multi-basepath hosts, no match, case-insensitive hostnames, and paging
• Manual testing completed — not run against a live preview (would require deleting a real deployment); pnpm --filter @fern-api/cli exec vitest run src/commands/docs-preview passes, pnpm turbo run compile --filter @fern-api/cli and pnpm lint:biome clean

Depends on nothing, but pairs with fern-api/fern-platform#14208, which stops a deleted site from continuing to serve its statically-cached HTML.

Link to Devin session: https://app.devin.ai/sessions/8b998ca2cd694ebca3ac34947854002c

---

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ 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

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 1 additional finding.