Improve @ file search: include specific hidden dirs such as .github, .gitlab (#2981)

# Improve @ file search: include specific hidden dirs

This should close #2980

## What
- Extend `@` fuzzy file search to include select top-level hidden
directories:
`.github`, `.gitlab`, `.circleci`, `.devcontainer`, `.azuredevops`,
`.vscode`, `.cursor`.
- Keep all other hidden directories excluded to avoid noise and heavy
traversals.

## Why
- Common project config lives under these dot-dirs (CI, editor,
devcontainer); users expect `@.github/...` and similar paths to resolve.
- Prior behavior hid all dot-dirs, making these files undiscoverable.

## How
- In `codex-file-search` walker:
  - Enable hidden entries via `WalkBuilder.hidden(false)`.
- Add `filter_entry` to only allow those specific root dot-directories;
other hidden paths remain filtered out.
  - Preserve `.gitignore` semantics and existing exclude handling.

## Local checks
- Ran formatting: `just fmt`
- Ran lint (scoped): `just fix -p codex-file-search`
- Ran tests:
  - `cargo test -p codex-file-search`
  - `cargo test -p codex-tui`

## Readiness
- Branch is up-to-date locally; tests pass; lint/format applied.
- No merge conflicts expected.
- Marking Ready for review.

---------

Signed-off-by: lionelchg <lionel.cheng@hotmail.fr>

codex-rs/file-search/src/lib.rs

@@ -151,7 +151,13 @@ pub fn run(
     // Use the same tree-walker library that ripgrep uses. We use it directly so
     // that we can leverage the parallelism it provides.
     let mut walk_builder = WalkBuilder::new(search_directory);
-    walk_builder.threads(num_walk_builder_threads);
+    walk_builder
+        .threads(num_walk_builder_threads)
+        // Allow hidden entries.
+        .hidden(false)
+        // Don't require git to be present to apply to apply git-related ignore rules.
+        .require_git(false);
+
     if !exclude.is_empty() {
         let mut override_builder = OverrideBuilder::new(search_directory);
         for exclude in exclude {