Assemble sandbox/approval/network prompts dynamically (#8961)

- Add a single builder for developer permissions messaging that accepts
SandboxPolicy and approval policy. This builder now drives the developer
“permissions” message that’s injected at session start and any time
sandbox/approval settings change.
- Trim EnvironmentContext to only include cwd, writable roots, and
shell; removed sandbox/approval/network duplication and adjusted XML
serialization and tests accordingly.

Follow-up: adding a config value to replace the developer permissions
message for custom sandboxes.

codex-rs/protocol/BUILD.bazel

@@ -3,4 +3,5 @@ load("//:defs.bzl", "codex_rust_crate")
 codex_rust_crate(
     name = "protocol",
     crate_name = "codex_protocol",
+    compile_data = glob(["src/prompts/permissions/**/*.md"]),
 )