fix(app delete): reject positional app names

[KirtiRamchandani]

Rejects positional application names for copilot app delete so copilot app delete example-app fails before it can delete the default workspace application. The command examples and command reference now show --name for deleting a specific application.

Fixes #6048

Tests run:

• go test ./internal/pkg/cli -run TestAppDeleteCommandRejectsPositionalAppName
• go test ./internal/pkg/cli -run 'Test(AppDeleteCommandRejectsPositionalAppName|DeleteAppOpts_(Ask|Execute))'
• git diff --check

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[KirtiRamchandani]

The failing Summary / Mergify Merge Queue checks here are from the repository-wide Mergify config, not this PR's code. The check-run output reports actions.queue.method and actions.queue.commit_message_template as invalid extra inputs.

Opened #7398 to move the squash merge strategy and commit-message settings onto queue_rules, which should address the shared Mergify validation failure.