Honor the parent `models auth --agent <id>` flag across auth write commands: `add`, `login`, `setup-token`, `paste-token`, and `login-github-copilot`.
The auth helpers now resolve the requested configured agent before choosing the auth-profile store and provider workspace, while preserving default-agent behavior when `--agent` is omitted.
Validation:
- `pnpm test src/cli/models-cli.test.ts src/commands/models/auth.test.ts`
- `pnpm test src/commands/models/auth.test.ts`
- `pnpm docs:check-mdx`
- `pnpm check:changed`
- `pnpm check`
- `pnpm build`
- `pnpm test src/cli/run-main.test.ts`
Full `pnpm test` was also run; it failed in unrelated `src/cli/run-main.test.ts` assertions during the full-suite order, while the exact file passes on both latest main and this branch. The PR diff only touches models auth CLI/auth files, docs, and changelog.
Fixes#71864.
Thanks @neeravmakwana.
