Document or support fallback when semantic search is unavailable in a repository region

Summary

entire checkpoint search cannot search a repository whose data is hosted in an unsupported region. Entire CLI 0.9.0 now returns a clear region-availability error, but the documented search workflow does not describe regional availability or a supported local fallback.

Environment

• Entire CLI: 0.9.0
• OS: Windows amd64
• Authentication: active GitHub device-flow login
• Repository: Entire-enabled Git repository with local and remote entire/checkpoints/v1 refs

Reproduction

Run an exact search for a known session that exists in local checkpoint history:

entire checkpoint search "<known-session-id>" --json

Actual result:

search failed: semantic search: semantic search is not yet available in the region(s) hosting this search

The same session is available locally:

entire checkpoint explain --session "<known-session-id>" --search-all --json --limit 1000 --no-pager

This returns multiple checkpoints. The default branch-limited local lookup can return zero for the same older session.

Expected Behavior

One of the following should be supported and documented:

1. Hosted semantic search is available for every region where Entire can host/index repository checkpoint data; or
2. The CLI provides a supported local-history fallback when hosted search is unavailable; or
3. The search documentation clearly lists regional availability and the exact local fallback command.

Machine-readable --json calls should ideally return a structured error that distinguishes regional unavailability from an empty search result.

Additional Context

On Entire CLI 0.8.42, the same exact query returned an empty result set, which could be misinterpreted as absent history. Version 0.9.0 improves the error message, but agents still need a documented way to avoid a false no-history conclusion.

Acceptance Criteria

• Regional search availability is documented or made universal.
• Automation can distinguish regional unavailability from zero matches.
• A supported fallback is documented when hosted search cannot serve the repository.

Start with the entire checkpoint search and entire checkpoint explain --session ... --search-all --json entry points, reproducing the regional error and local-history result described here. Read the search documentation and the JSON error handling around these commands. Done means regional availability and a supported fallback are documented or implemented, with machine-readable output distinguishing unavailability from zero matches.