guardian initial feedback / tweaks (#13897)

## Summary
- remove the remaining model-visible guardian-specific `on-request`
prompt additions so enabling the feature does not change the main
approval-policy instructions
- neutralize user-facing guardian wording to talk about automatic
approval review / approval requests rather than a second reviewer or
only sandbox escalations
- tighten guardian retry-context handling so agent-authored
`justification` stays in the structured action JSON and is not also
injected as raw retry context
- simplify guardian review plumbing in core by deleting dead
prompt-append paths and trimming some request/transcript setup code

## Notable Changes
- delete the dead `permissions/approval_policy/guardian.md` append path
and stop threading `guardian_approval_enabled` through model-facing
developer-instruction builders
- rename the experimental feature copy to `Automatic approval review`
and update the `/experimental` snapshot text accordingly
- make approval-review status strings generic across shell, patch,
network, and MCP review types
- forward real sandbox/network retry reasons for shell and unified-exec
guardian review, but do not pass agent-authored justification as raw
retry context
- simplify `guardian.rs` by removing the one-field request wrapper,
deduping reasoning-effort selection, and cleaning up transcript entry
collection

## Testing
- `just fmt`
- full validation left to CI

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/protocol/src/prompts/permissions/approval_policy/guardian.md

@@ -1,3 +0,0 @@
-Guardian approvals are enabled. While `approval_policy` is still `on-request`, approval prompts are routed to a guardian subagent instead of the user. Use `sandbox_permissions: "require_escalated"` with a concise `justification` when you need unsandboxed execution, and use `sandbox_permissions: "with_additional_permissions"` plus `additional_permissions` when you need broader sandboxed access. Codex will ask the guardian subagent to assess the risk automatically.
-
-Do not message the user before requesting escalation. If the guardian rejects an action, do not attempt the same outcome via workaround, indirect execution, or policy circumvention. Either choose a materially safer alternative or stop and ask the user for guidance.