runner: stage attached files into per-run cwd, drop broken --files arg

The old code passed `--files <abspath>` to `claude -p`, but the CLI has
no such flag — it errored out with "unknown option '--files'" on the
first /run call carrying a file. The flag had been there since the
file-attachment path was added and was apparently never exercised end
to end against current claude versions.

New behavior: each file_path is hardlinked (cross-fs falls back to
copy) into the per-run cwd under a stable `attachment-N.<ext>` name,
and the prompt is appended with a `Attached files in the current
directory:` manifest pointing at the relative paths. claude -p's Read
tool then loads each file on demand — including image content for
vision models.

acpx /sessions path unaffected — it has its own _format_prompt_with_files.

clawdforge/runner.py

@@ -63,9 +63,6 @@ class Runner:
                 cmd += ["--append-system-prompt", system]
         if effort:
             cmd += ["--effort", effort]
-        if files:
-            for f in files:
-                cmd += ["--files", f]
 
         timeout = timeout_secs or self.default_timeout
 
@@ -74,6 +71,41 @@ class Runner:
         cwd = Path(self.runs_dir) / run_id
         cwd.mkdir(parents=True, exist_ok=True)
 
+        # File attachments are surfaced to claude by hardlinking (or
+        # copying, cross-fs) them into the per-run cwd under stable
+        # names `attachment-N.<ext>`, then appending a short manifest to
+        # the prompt that references them by relative path. claude -p's
+        # built-in Read tool then loads each on demand — including image
+        # content for vision models.
+        #
+        # Earlier impl tried `--files` / `--file` flags on the CLI but
+        # both are claude.ai-hosted file_id forms, not local-path
+        # accepting; pure-CWD staging is the only sane local path.
+        attachment_lines: list[str] = []
+        if files:
+            for idx, src in enumerate(files, start=1):
+                src_p = Path(src)
+                ext = src_p.suffix.lstrip(".") or "bin"
+                staged_name = f"attachment-{idx}.{ext}"
+                staged_path = cwd / staged_name
+                try:
+                    os.link(src, staged_path)
+                except OSError:
+                    # Cross-filesystem hardlinks fail with EXDEV; fall back to copy.
+                    shutil.copy2(src, staged_path)
+                attachment_lines.append(f"- ./{staged_name}")
+        if attachment_lines:
+            manifest = (
+                "\n\nAttached files in the current directory (use the Read tool to view):\n"
+                + "\n".join(attachment_lines)
+            )
+            if use_stdin:
+                # Mutate the input that goes to stdin.
+                prompt = prompt + manifest
+            else:
+                # Mutate the inline-positional arg we already pushed.
+                cmd[2] = cmd[2] + manifest
+
         started = time.monotonic()
         try:
             proc = subprocess.run(