Add file browser, tests, and bin entrypoint

README.md

@@ -27,6 +27,34 @@ fido /path/to/project   # work on a specific directory
 
 Set `OPENROUTER_API_KEY` (and optionally `TAVILY_API_KEY`) in `~/.fido/.env`.
 
+### File mentions
+
+While typing a message, press **`Ctrl+O`** to open an in-prompt file browser.
+It is `.gitignore`-aware and hides dot-directories, just like `list_files`.
+
+| Key         | Action                                             |
+| ----------- | -------------------------------------------------- |
+| `↑` / `↓`   | Move the highlight                                 |
+| `Enter`     | Descend into a directory, or open the line selector|
+| `Tab`       | Mention the highlighted file (whole file)          |
+| `Backspace` | Go to the parent directory                         |
+| `Esc`       | Cancel back to the prompt                          |
+
+In the line selector, `↑`/`↓` move the cursor, `v` starts/ends a range, `g`/`G`
+jump to the first/last line, and `Enter` inserts the mention.
+
+The mention is plain text spliced in at the cursor; the model reads the file
+itself via `read_file` (it is a *reference*, not embedded contents):
+
+| Selection   | Inserted text           |
+| ----------- | ----------------------- |
+| Whole file  | `@src/fido.js`          |
+| Line range  | `@src/fido.js:120-145`  |
+| Single line | `@src/fido.js:120`      |
+
+Paths are relative to the workspace root with forward slashes; line numbers are
+1-based (the model maps them to `read_file`'s 0-indexed `seek`).
+
 ### Layout
 
 ```