refactor: centralize filesystem permissions precedence (#14174)

## Stack

   fix: fail closed for unsupported split windows sandboxing #14172
   fix: preserve split filesystem semantics in linux sandbox #14173
   fix: align core approvals with split sandbox policies #14171
-> refactor: centralize filesystem permissions precedence #14174

## Summary
- add a shared per-path split filesystem precedence helper in
`FileSystemSandboxPolicy`
- derive readable, writable, and unreadable roots from the same
most-specific resolution rules
- add regression coverage for nested `write` / `read` / `none` carveouts
and legacy bridge enforcement detection

## Testing
- cargo test -p codex-protocol
- cargo clippy -p codex-protocol --tests -- -D warnings

codex-rs/core/config.schema.json

@@ -594,10 +594,11 @@
       "type": "object"
     },
     "FileSystemAccessMode": {
+      "description": "Access mode for a filesystem entry.\n\nWhen two equally specific entries target the same path, we compare these by conflict precedence rather than by capability breadth: `none` beats `write`, and `write` beats `read`.",
       "enum": [
-        "none",
         "read",
-        "write"
+        "write",
+        "none"
       ],
       "type": "string"
     },