fix(core): default CODEX_HOME to platform config dir with legacy ~/.codex fallback

[tomascupr]

This PR updates Codex's default state directory to follow platform conventions while preserving backward compatibility.

Summary

• Precedence:
  1. If CODEX_HOME is set and non‑empty, use it (canonicalized).
  2. Else, if legacy ~/.codex exists, continue using it.
  3. Else, default to dirs::config_dir()/codex (XDG on Linux; ~/Library/Application Support/codex on macOS; %AppData%\codex on Windows).
• No change to the rest of the code: everything uses config.codex_home as before.

Why

• Addresses Change the hardcoded $HOME/.codex path #4407: reduces $HOME clutter and aligns with XDG/AppData/Library conventions.
• Keeps existing users stable via legacy fallback; new installs adopt the modern default seamlessly.

Details

• Code: codex-rs/core/src/config.rs::find_codex_home() implements the new resolution order.
• Tests: add unit tests covering env override, legacy fallback, and platform defaults.
• Docs: replace hardcoded ~/.codex references with $CODEX_HOME and document platform defaults + legacy fallback.

Compatibility & Migration

• Existing users with ~/.codex are unaffected (explicitly preferred).
• New installs default under the platform config directory unless CODEX_HOME is set.

Verification

• Ran just fmt.
• Ran just fix -p codex-core.
• Tests:
  • cargo test -p codex-core → PASS
  • cargo test --all-features (workspace) → PASS

Closes #4407.