Deprecate TurnContext cwd and resolve_path (#22519)

## Why

`TurnContext::cwd` and `TurnContext::resolve_path` are being phased out
in favor of using the selected turn environment cwd directly.
Deprecating both APIs makes any new direct dependency visible while
preserving the existing migration path for current callers.

## What Changed

- Marked `TurnContext::cwd` and `TurnContext::resolve_path` as
deprecated with guidance to use the selected turn environment cwd
instead.
- Added exact `#[allow(deprecated)]` suppressions at each existing
direct usage site, including tests, rather than adding crate-wide
suppression.
- Kept the change behavior-preserving: current cwd reads, writes, and
path resolution continue to use the same values.

## Verification

- `just fmt`
- `cargo check -p codex-core`
- `cargo check -p codex-core --tests`
- `git diff --check`

codex-rs/core/src/codex_delegate_tests.rs

@@ -207,6 +207,7 @@ async fn handle_request_permissions_uses_tool_call_id_for_round_trip() {
         scope: PermissionGrantScope::Turn,
         strict_auto_review: false,
     };
+    #[allow(deprecated)]
     let delegated_cwd = parent_ctx.cwd.join("delegated-cwd");
     let cancel_token = CancellationToken::new();
     let request_call_id = call_id.clone();