cli: rename profile v2 flag to --profile (#23883)

## Why

Profile v2 is taking over the user-facing profile selection path, so the
CLI no longer needs to expose the transitional `--profile-v2` spelling.
This switches the public args surface to `--profile` before the
remaining legacy profile plumbing is removed separately.

## What

- Rebind `--profile` and `-p` to the v2 profile name argument that
selects `$CODEX_HOME/<name>.config.toml`.
- Stop parsing the legacy shared CLI profile argument while keeping its
implementation path in place for follow-up cleanup.
- Update CLI validation, profile-name parse errors, and the
legacy-profile collision message/tests to refer to `--profile`.

## Testing

- `cargo test -p codex-cli -p codex-config -p codex-protocol -p
codex-utils-cli`

codex-rs/protocol/src/config_types.rs

@@ -114,7 +114,7 @@ impl fmt::Display for ProfileV2NameParseError {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(
             f,
-            "invalid --profile-v2 value `{}`; pass a plain name such as `work`",
+            "invalid --profile value `{}`; pass a plain name such as `work`",
             self.value
         )
     }