feat: @mention files in codex (#701)

Solves #700

## State of the World Before

Prior to this PR, when users wanted to share file contents with Codex,
they had two options:
- Manually copy and paste file contents into the chat
- Wait for the assistant to use the shell tool to view the file

The second approach required the assistant to:
1. Recognize the need to view a file
2. Execute a shell tool call
3. Wait for the tool call to complete
4. Process the file contents

This consumed extra tokens and reduced user control over which files
were shared with the model.

## State of the World After

With this PR, users can now:
- Reference files directly in their chat input using the `@path` syntax
- Have file contents automatically expanded into XML blocks before being
sent to the LLM

For example, users can type `@src/utils/config.js` in their message, and
the file contents will be included in context. Within the terminal chat
history, these file blocks will be collapsed back to `@path` format in
the UI for clean presentation.

Tag File suggestions:
<img width="857" alt="file-suggestions"
src="https://github.com/user-attachments/assets/397669dc-ad83-492d-b5f0-164fab2ff4ba"
/>

Tagging files in action:
<img width="858" alt="tagging-files"
src="https://github.com/user-attachments/assets/0de9d559-7b7f-4916-aeff-87ae9b16550a"
/>

Demo video of file tagging:
[![Demo video of file
tagging](https://img.youtube.com/vi/vL4LqtBnqt8/0.jpg)](https://www.youtube.com/watch?v=vL4LqtBnqt8)

## Implementation Details

This PR consists of 2 main components:

1. **File Tag Utilities**:
- New `file-tag-utils.ts` utility module that handles both expansion and
collapsing of file tags
- `expandFileTags()` identifies `@path` tokens and replaces them with
XML blocks containing file contents
- `collapseXmlBlocks()` reverses the process, converting XML blocks back
to `@path` format for UI display
- Tokens are only expanded if they point to valid files (directories are
ignored)
   - Expansion happens just before sending input to the model

2. **Terminal Chat Integration**:
- Leveraged the existing file system completion system for tabbing to
support the `@path` syntax
   - Added `updateFsSuggestions` helper to manage filesystem suggestions
- Added `replaceFileSystemSuggestion` to replace input with filesystem
suggestions
- Applied `collapseXmlBlocks` in the chat response rendering so that
tagged files are shown as simple `@path` tags

The PR also includes test coverage for both the UI and the file tag
utilities.

## Next Steps

Some ideas I'd like to implement if this feature gets merged:

- Line selection: `@path[50:80]` to grab specific sections of files
- Method selection: `@path#methodName` to grab just one function/class
- Visual improvements: highlight file tags in the UI to make them more
noticeable

codex-cli/tests/text-buffer.test.ts

@@ -136,6 +136,33 @@ describe("TextBuffer – basic editing parity with Rust suite", () => {
     });
   });
 
+  describe("cursor initialization", () => {
+    it("initializes cursor to (0,0) by default", () => {
+      const buf = new TextBuffer("hello\nworld");
+      expect(buf.getCursor()).toEqual([0, 0]);
+    });
+
+    it("sets cursor to valid position within line", () => {
+      const buf = new TextBuffer("hello", 2);
+      expect(buf.getCursor()).toEqual([0, 2]); // cursor at 'l'
+    });
+
+    it("sets cursor to end of line", () => {
+      const buf = new TextBuffer("hello", 5);
+      expect(buf.getCursor()).toEqual([0, 5]); // cursor after 'o'
+    });
+
+    it("sets cursor across multiple lines", () => {
+      const buf = new TextBuffer("hello\nworld", 7);
+      expect(buf.getCursor()).toEqual([1, 1]); // cursor at 'o' in 'world'
+    });
+
+    it("defaults to position 0 for invalid index", () => {
+      const buf = new TextBuffer("hello", 999);
+      expect(buf.getCursor()).toEqual([0, 0]);
+    });
+  });
+
   /* ------------------------------------------------------------------ */
   /*  Vertical cursor movement – we should preserve the preferred column  */
   /* ------------------------------------------------------------------ */