fix(extensions): remove colliding -i short flag from ext:migrate --ext-instance option

[shettyvarun268]

Description

In firebase ext:migrate, the --ext-instance <instanceId> option was registered with the short flag -i. However, -i, --interactive is globally registered across all CLI commands in src/index.ts.

Because Commander processes global options first, passing -i <instanceId> caused Commander to parse -i as the boolean --interactive flag, leaving the instance ID unconsumed or causing unexpected prompt behaviors/errors in scripted environments. Furthermore, --help actively advertised the broken -i short flag.

This PR removes the colliding -i short flag from ext:migrate, making --ext-instance <instanceId> the unambiguous option.

Scenarios Tested

• Verified firebase ext:migrate --help displays --ext-instance <instanceId> without the conflicting -i shortcut.
• Ran unit tests via mocha: npm test -- src/extensions/migrate.spec.ts (38/38 passing).
• Verified migration runs cleanly with --ext-instance <instanceId>.

Sample Commands

firebase ext:migrate --ext-instance storage-resize-images --project my-project

[gemini-code-assist]

Code Review

This pull request removes the short option -i from the --ext-instance option in the ext:migrate command. I have no feedback to provide as there are no review comments and the change is straightforward.