fix(core): make JSON-to-SQLite migration truly one-time

[vereis]

Issue for this PR

Closes #16885

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

The startup JSON->SQLite migration gate currently checks for ~/.local/share/opencode/opencode.db. On non-latest channels, the active DB can be channel-specific (for example opencode-local.db), so that filename check can stay false forever and migration reruns every launch.

This change switches the gate to a dedicated marker file at ~/.local/share/opencode/storage/json-sqlite-migration.done and writes that marker after a successful migration run. That makes the migration truly one-time regardless of DB filename/channel.

Why this approach instead of checking channel DB names: checking channel DB filenames only fixes the rerun case, but not the skip case where a DB file exists from prior schema migrations even though JSON->SQLite import never completed. A dedicated migration-completion marker tracks the actual event we care about.

How did you verify your code works?

• Reproduced repeated migration locally before the change (migration banner showed on each startup)
• Applied this patch and validated migration no longer reruns after completion
• Ran typecheck locally using nix-provided bun:
  • nix shell nixpkgs#bun --command bun install
  • nix shell nixpkgs#bun --command bun run --cwd packages/opencode typecheck

Screenshots / recordings

Before:

After:

N/A (CLI startup behavior change, no UI change)

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍