fix: [PR-1797] suppress migration banner after sf migrate completes

[sigmachirality]

Summary

• Persist migrated_to_rust_cli: true in ~/.sfcompute/config when sf migrate succeeds.
• Gate the migration banner on that flag so sf-old stops nagging users who've already migrated.
• SF_CLI_DISABLE_MIGRATE_BANNER continues to work as an explicit override.

Test plan

• Run sf migrate on a clean config; verify migrated_to_rust_cli: true is written.
• Re-run sf-old <anything> and confirm the migration banner no longer renders.
• With the flag absent, banner still appears as before.

🤖 Generated with Claude Code

[semanticdiff-com]

Review changes with  

Changed Files

File	Status
src/index.ts	49% smaller
src/helpers/config.ts	0% smaller
src/lib/migrate.ts	0% smaller

[indent]

PR Summary

Stops the legacy sf-old CLI from repeatedly showing the "we've rewritten sf in Rust" banner to users who already ran sf migrate, by persisting a flag in ~/.sfcompute/config and gating the banner on it.

• src/helpers/config.ts: adds optional migrated_to_rust_cli?: boolean to the Config interface.
• src/lib/migrate.ts: after a successful install (or dev-mode skip), best-effort writes { ...config, migrated_to_rust_cli: true } via saveConfig, wrapped in try/catch so a write failure just leaves the banner showing.
• src/index.ts: hoists the existing loadConfig() call above the banner block (net I/O unchanged — the prior call site is removed and the account-id hydration reuses the hoisted config), and adds !config.migrated_to_rust_cli as a new conjunct on the banner-show condition alongside the existing SF_CLI_DISABLE_MIGRATE_BANNER / migrate / upgrade guards.

Issues

No issues found.

CI Checks

All CI checks passed on 7e8040d.

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