chore: remove legacy review share-base URL inference after migration window #783

Context

Multi-instance sharing persists share_base_url in new and mutated review files. Review files created before that field existed temporarily infer their originating instance from configured target URL boundaries.

Removal criteria

• Keep inference through a documented compatibility window of at least two minor releases after multi-instance sharing ships.
• Ensure ordinary share-state mutations opportunistically backfill share_base_url, including explicit confirmation for ambiguous legacy files.
• Document a manual migration/recovery path for reviews whose original target is no longer configured.
• Before removal, confirm support reports and repository examples indicate legacy files are sufficiently rare.
• Remove the inference code and its compatibility tests only after the window and backfill criteria are met; never replace ambiguous inference with the current default target.

Cleanup scope

• Remove legacy root-host/path-prefix inference.
• Remove the temporary ambiguity confirmation path where no longer needed.
• Retain direct use of persisted share_base_url and missing-target warnings.