## Why
`profile-v2` layers the selected profile file on top of the base user
`config.toml`, but the legacy `[profiles]` table also stores named
profile overrides in that same base file. Allowing both paths during one
load makes it too easy to get a mixed profile where stale legacy
settings still influence a profile-v2 run.
## What Changed
- Detect a legacy `[profiles]` table in the base user config whenever
`--profile-v2` selects a profile file.
- Fail config loading with an `InvalidData` error that tells the user to
move those settings into the selected profile-v2 file or remove
`[profiles]`.
- Add a loader regression covering `--profile-v2` with legacy
`[profiles]` in `config.toml`.
## Testing
- `cargo test -p codex-config
profile_v2_rejects_legacy_profiles_in_base_user_config`