fix: codex-arg0 no longer depends on codex-core (#12434)

## Why

`codex-rs/arg0` only needed two things from `codex-core`:

- the `find_codex_home()` wrapper
- the special argv flag used for the internal `apply_patch`
self-invocation path

That made `codex-arg0` depend on `codex-core` for a very small surface
area. This change removes that dependency edge and moves the shared
`apply_patch` invocation flag to a more natural boundary
(`codex-apply-patch`) while keeping the contract explicitly documented.

## What Changed

- Moved the internal `apply_patch` argv[1] flag constant out of
`codex-core` and into `codex-apply-patch`.
- Renamed the constant to `CODEX_CORE_APPLY_PATCH_ARG1` and documented
that it is part of the Codex core process-invocation contract (even
though it now lives in `codex-apply-patch`).
- Updated `arg0`, the core apply-patch runtime, and the `codex-exec`
apply-patch test to import the constant from `codex-apply-patch`.
- Updated `codex-rs/arg0` to call
`codex_utils_home_dir::find_codex_home()` directly instead of
`codex_core::config::find_codex_home()`.
- Removed the `codex-core` dependency from `codex-rs/arg0` and added the
needed direct dependency on `codex-utils-home-dir`.
- Added `codex-apply-patch` as a dev-dependency for `codex-rs/exec`
tests (the apply-patch test now imports the moved constant directly).

## Verification

- `cargo test -p codex-apply-patch`
- `cargo test -p codex-arg0`
- `cargo test -p codex-core --lib apply_patch`
- `cargo test -p codex-exec
test_standalone_exec_cli_can_use_apply_patch`
- `cargo shear`

codex-rs/exec/tests/suite/apply_patch.rs

@@ -2,7 +2,7 @@
 
 use anyhow::Context;
 use assert_cmd::prelude::*;
-use codex_core::CODEX_APPLY_PATCH_ARG1;
+use codex_apply_patch::CODEX_CORE_APPLY_PATCH_ARG1;
 use core_test_support::responses::ev_apply_patch_custom_tool_call;
 use core_test_support::responses::ev_apply_patch_function_call;
 use core_test_support::responses::ev_completed;
@@ -24,7 +24,7 @@ fn test_standalone_exec_cli_can_use_apply_patch() -> anyhow::Result<()> {
     fs::write(&absolute_path, "original content\n")?;
 
     Command::new(codex_utils_cargo_bin::cargo_bin("codex-exec")?)
-        .arg(CODEX_APPLY_PATCH_ARG1)
+        .arg(CODEX_CORE_APPLY_PATCH_ARG1)
         .arg(
             r#"*** Begin Patch
 *** Update File: source.txt