core/cli+config: attach files to a turn with @path mentions

Typing `@shot.png` in the line attaches that picture to the turn — no separate
command — and `@notes.txt` appends the file as text under a `--- @path ---`
header, capped like a tool result with a resumable `truncate_token`.
`@"my shot.png"` is the quoted form, and Tab completes the path after `@`
(directories end in `/`, a name with a space completes quoted).

An image is skipped, with a note naming the key to set, on a model without
`image_in`; a missing or denylisted path is a note too, and the turn still goes.
The default catalogs mark what can take a picture (`deepseek-flash` yes,
`deepseek-v4-pro` no, the OpenAI defaults yes).

`Agent.run` takes the parts, so the turn is stored with its images and shows up
as `[image]` wherever only prose is wanted.

tests/test_cli/test_readline_setup.py

@@ -170,6 +170,46 @@ def test_completer_stream_on_off(tmp_path: object, monkeypatch: object) -> None:
     assert completer("o", 1) == "off"
 
 
+def test_completer_path_mentions(tmp_path: object, monkeypatch: object) -> None:
+    """An ``@path`` token completes files, wherever it sits in the line."""
+    import readline
+    from pathlib import Path
+
+    import pytest
+
+    assert isinstance(tmp_path, Path)
+    assert isinstance(monkeypatch, pytest.MonkeyPatch)
+    (tmp_path / "shot.png").write_bytes(b"")
+    (tmp_path / "screens").mkdir()
+
+    state = _minimal_state(tmp_path)
+    completer = build_completer(state)
+    monkeypatch.setattr(readline, "get_line_buffer", lambda: "look at @sh")
+    monkeypatch.setattr(readline, "get_begidx", lambda: len("look at "))
+    monkeypatch.chdir(tmp_path)
+
+    found = [item for index in range(10) if (item := completer("@sh", index)) is not None]
+    assert found == ["@shot.png"]
+
+
+def test_a_plain_word_is_still_a_slash_argument(tmp_path: object, monkeypatch: object) -> None:
+    """The mention branch must not swallow ordinary argument completion."""
+    import readline
+    from pathlib import Path
+
+    import pytest
+
+    assert isinstance(tmp_path, Path)
+    assert isinstance(monkeypatch, pytest.MonkeyPatch)
+
+    state = _minimal_state(tmp_path)
+    completer = build_completer(state)
+    monkeypatch.setattr(readline, "get_line_buffer", lambda: "/provider r")
+    monkeypatch.setattr(readline, "get_begidx", lambda: len("/provider "))
+
+    assert completer("r", 0) == "remote"
+
+
 def test_complete_slash_args_from_registry(tmp_path: object) -> None:
     from pathlib import Path