Singular projectHostnameBase (drop projectHostnameBases array)

Problem#

AppConfig.projectHostnameBases is string[] and Doppler exposes APP_CONFIG_PROJECT_HOSTNAME_BASES as a JSON array, e.g. ["iterate.app"].

Every environment uses exactly one base:

Nothing passes multiple bases in production. Call sites already treat it as singular: projectHostnameBases[0] (DNS, MCP URLs, settings UI, oRPC), and the flatMap in the wrangler-config generator only matters when it registers base + *.base routes for that one entry.

The array shape is leftover from an earlier “maybe multiple bases per deploy” idea. It adds noise ([0], JSON array in Doppler, plural naming) without behavior we use. Custom apex hosts (iterate.com) are custom_hostname on a project, not an extra entry in this list — see docs/devops-cloudflare-doppler.md.

Goal#

One required string per environment:

projectHostnameBase: publicValue(z.string().trim().min(1));

Env: APP_CONFIG_PROJECT_HOSTNAME_BASE=iterate.app (plain string, not JSON array).

Scope#

• apps/os/src/config.ts — schema + rename
• apps/os/scripts/generate-wrangler-config.ts — project route hostnames for the base
• Ingress / routing: project-host-routing.ts, worker entrypoints, project-platform-host-routing.ts, project-durable-object.ts
• oRPC context, auth.ts, UI (settings, app-sidebar, project list)
• apps/os/scripts/cli.ts (claude-mcp), apps/os/scripts/sync-auth-clients.ts
• scripts/preview/preview.ts parse helpers
• Tests: replace array fixtures with a single string
• Doppler: migrate all os configs from JSON array to string (prd, previewN, dev*)
• Docs: docs/devops-cloudflare-doppler.md, apps/os/docs/architecture-and-operations.md, task files that mention the old name

Non-goals#

• Changing how manual iterate.com routes work
• Merging baseUrl and projectHostnameBase into one field (dashboard .com vs project .app zones stay separate)
• Supporting multiple bases again without a deliberate design pass

Acceptance#

• No projectHostnameBases / APP_CONFIG_PROJECT_HOSTNAME_BASES in app code
• All Doppler os configs use APP_CONFIG_PROJECT_HOSTNAME_BASE string
• pnpm typecheck + apps/os tests pass
• Preview deploy still registers iterate-preview-N.app + *.iterate-preview-N.app routes