Bug

The JSON->SQLite startup migration can rerun on every launch for non-latest channels (for example local/dev builds).

Reproduction

1. Run OpenCode on a non-latest channel where the DB path is channel-specific (for example opencode-local.db)
2. Keep legacy JSON storage files under ~/.local/share/opencode/storage/
3. Start opencode twice

Actual

The startup banner "Performing one time database migration" appears on every launch and rescans/reimports JSON storage each time.

Expected

Migration should run once and skip on subsequent launches.

Root cause

The startup gate checks for ~/.local/share/opencode/opencode.db, but non-latest channels can use DB names like opencode-local.db, so the gate never flips to true.

Why not just check channel-specific DB names?

Checking channel DB filenames only addresses the rerun failure mode. It does not address the existing skip failure mode where a DB file can exist from prior schema migrations even if JSON->SQLite import has never completed. In that case, a DB-existence gate can incorrectly skip migration and orphan legacy JSON data.

Suggested fix

Use a dedicated migration completion marker (or migration table entry) instead of checking for a specific DB filename.