fix: diagnose custom providers unsupported by upstream TUI

[guanbear]

Summary

• distinguish configured headless model access from provider IDs accepted by the upstream TUI login gate
• fail early with an actionable zai / bigmodel provider-ID diagnostic instead of opening a TUI that reports no model access
• document the compatibility constraint and cover both compatible and custom provider configurations

Why

A valid custom provider can run zcode --prompt, while the upstream interactive TUI still rejects it because its direct-key gate only recognizes zai and bigmodel. The launcher previously treated both capabilities as equivalent, which made integrations believe an interactive worker was ready when it was not.

This change does not hard-code a release version and does not expose API keys.

Verification

• npx --yes bun run typecheck
• full suite: 491 passed; one unrelated 5s rich-Markdown performance test timed out at 5.17s
• isolated rerun: npx --yes bun test test/code-highlighter.test.ts --timeout 10000 (16 passed)
• all launcher/runtime integration failures pass after npx --yes bun run sync:local

[guanbear]

Closing after re-validating the real integration. The affected local setup now uses the official provider ID, so this custom-provider diagnostic does not address the active ZCode/Orca issue. The remaining fix belongs in Orca's interactive launch and process recognition path. Thanks.

[guanbear]

Correction: the official provider ID in the current setup is bigmodel.