feat: clipboard image paste in CLI (Cmd+V / Ctrl+V)

Copy an image to clipboard (screenshot, browser, etc.) and paste into
the Hermes CLI. The image is saved to ~/.hermes/images/, shown as a
badge above the input ([📎 Image #1]), and sent to the model as a
base64-encoded OpenAI vision multimodal content block.

Implementation:
- hermes_cli/clipboard.py: clean module with platform-specific extraction
  - macOS: pngpaste (if installed) → osascript fallback (always available)
  - Linux: xclip (apt install xclip)
- cli.py: BracketedPaste key handler checks clipboard on every paste,
  image bar widget shows attached images, chat() converts to multimodal
  content format, Ctrl+C clears attachments

Inspired by @m0at's fork (https://github.com/m0at/hermes-agent) which
implemented image paste support for local vision models. Reimplemented
cleanly as a separate module with tests.

tests/tools/test_clipboard.py

@@ -0,0 +1,107 @@
+"""Tests for hermes_cli/clipboard.py — clipboard image extraction."""
+
+import subprocess
+import sys
+from pathlib import Path
+from unittest.mock import patch, MagicMock
+
+import pytest
+
+from hermes_cli.clipboard import (
+    save_clipboard_image,
+    _linux_save,
+    _macos_pngpaste,
+    _macos_osascript,
+)
+
+
+class TestSaveClipboardImage:
+    """Platform dispatch."""
+
+    def test_dispatches_to_macos_on_darwin(self, tmp_path):
+        dest = tmp_path / "out.png"
+        with patch("hermes_cli.clipboard.sys") as mock_sys:
+            mock_sys.platform = "darwin"
+            with patch("hermes_cli.clipboard._macos_save", return_value=False) as mock_mac:
+                save_clipboard_image(dest)
+                mock_mac.assert_called_once_with(dest)
+
+    def test_dispatches_to_linux_on_linux(self, tmp_path):
+        dest = tmp_path / "out.png"
+        with patch("hermes_cli.clipboard.sys") as mock_sys:
+            mock_sys.platform = "linux"
+            with patch("hermes_cli.clipboard._linux_save", return_value=False) as mock_linux:
+                save_clipboard_image(dest)
+                mock_linux.assert_called_once_with(dest)
+
+    def test_creates_parent_dirs(self, tmp_path):
+        dest = tmp_path / "deep" / "nested" / "out.png"
+        with patch("hermes_cli.clipboard.sys") as mock_sys:
+            mock_sys.platform = "linux"
+            with patch("hermes_cli.clipboard._linux_save", return_value=False):
+                save_clipboard_image(dest)
+        assert dest.parent.exists()
+
+
+class TestMacosPngpaste:
+    def test_success(self, tmp_path):
+        dest = tmp_path / "out.png"
+        dest.write_bytes(b"fake png data")  # simulate pngpaste writing
+        with patch("hermes_cli.clipboard.subprocess.run") as mock_run:
+            mock_run.return_value = MagicMock(returncode=0)
+            assert _macos_pngpaste(dest) is True
+
+    def test_not_installed(self, tmp_path):
+        dest = tmp_path / "out.png"
+        with patch("hermes_cli.clipboard.subprocess.run", side_effect=FileNotFoundError):
+            assert _macos_pngpaste(dest) is False
+
+    def test_no_image_in_clipboard(self, tmp_path):
+        dest = tmp_path / "out.png"
+        with patch("hermes_cli.clipboard.subprocess.run") as mock_run:
+            mock_run.return_value = MagicMock(returncode=1)
+            assert _macos_pngpaste(dest) is False
+
+
+class TestMacosOsascript:
+    def test_no_image_type_in_clipboard(self, tmp_path):
+        dest = tmp_path / "out.png"
+        with patch("hermes_cli.clipboard.subprocess.run") as mock_run:
+            mock_run.return_value = MagicMock(
+                stdout="«class ut16», «class utf8»", returncode=0
+            )
+            assert _macos_osascript(dest) is False
+
+    def test_clipboard_info_check_fails(self, tmp_path):
+        dest = tmp_path / "out.png"
+        with patch("hermes_cli.clipboard.subprocess.run", side_effect=Exception("fail")):
+            assert _macos_osascript(dest) is False
+
+
+class TestLinuxSave:
+    def test_no_xclip_installed(self, tmp_path):
+        dest = tmp_path / "out.png"
+        with patch("hermes_cli.clipboard.subprocess.run", side_effect=FileNotFoundError):
+            assert _linux_save(dest) is False
+
+    def test_no_image_in_clipboard(self, tmp_path):
+        dest = tmp_path / "out.png"
+        with patch("hermes_cli.clipboard.subprocess.run") as mock_run:
+            mock_run.return_value = MagicMock(stdout="text/plain\n", returncode=0)
+            assert _linux_save(dest) is False
+
+    def test_image_in_clipboard(self, tmp_path):
+        dest = tmp_path / "out.png"
+        
+        def fake_run(cmd, **kwargs):
+            if "TARGETS" in cmd:
+                return MagicMock(stdout="image/png\ntext/plain\n", returncode=0)
+            # Extract call — write fake data
+            if "stdout" in kwargs and kwargs["stdout"]:
+                kwargs["stdout"].write(b"fake png")
+            return MagicMock(returncode=0)
+        
+        with patch("hermes_cli.clipboard.subprocess.run", side_effect=fake_run):
+            # Create the file to simulate xclip writing
+            dest.write_bytes(b"fake png")
+            assert _linux_save(dest) is True