fix: launch grok prompts interactively

[nikhilachale]

Summary

• launch Grok worker prompts as positional args instead of -p single-turn mode
• update Grok launch arg tests to assert normal prompts do not use -p / --single
• refresh Grok adapter comments to match the current interactive launch behavior

Closes #2516

Tests

• cd backend && go test ./internal/adapters/agent/grok
• cd backend && go test ./internal/adapters/agent/... (fails in unrelated internal/adapters/agent/kilocode: TestAuthStatusUnknownWhenKeyOnlyComesFromInteractiveShell times out; reproduced with go test ./internal/adapters/agent/kilocode)

Risks

• Low: preserves existing --no-auto-update, permission flags, and restore args; only changes fresh Grok prompt delivery.