## Why
External editor buffers can contain the current composer text and should not be
placed in directories exposed as writable by a restricted filesystem policy.
## What changed
- Create editor buffer files under a protected `editor` directory, trying the
configured Codex home, the default Codex home, and a workspace fallback.
- Reject candidates that overlap writable roots or resolve through symbolic
links, while preserving external editor support for full-disk-write policies.
- Surface an editor error when no protected directory is available.
## Testing
Add coverage for writable roots, aliases, symbolic links, fallback selection,
temporary-directory grants, full-disk-write policies, and editor invocation.
GitOrigin-RevId: df1029d81b327678991ab84ffd1077f0e5855631
