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.

hermes_cli/clipboard.py

@@ -0,0 +1,122 @@
+"""Clipboard image extraction for macOS and Linux.
+
+Provides a single function `save_clipboard_image(dest)` that checks the
+system clipboard for image data, saves it to *dest* as PNG, and returns
+True on success.  No external Python dependencies — uses only OS-level
+CLI tools that ship with the platform (or are commonly installed).
+
+Platform support:
+  macOS  — osascript (always available), pngpaste (if installed)
+  Linux  — xclip (apt install xclip)
+"""
+
+import logging
+import subprocess
+import sys
+from pathlib import Path
+
+logger = logging.getLogger(__name__)
+
+
+def save_clipboard_image(dest: Path) -> bool:
+    """Extract an image from the system clipboard and save it as PNG.
+
+    Returns True if an image was found and saved, False otherwise.
+    """
+    dest.parent.mkdir(parents=True, exist_ok=True)
+    if sys.platform == "darwin":
+        return _macos_save(dest)
+    return _linux_save(dest)
+
+
+# ── macOS ────────────────────────────────────────────────────────────────
+
+def _macos_save(dest: Path) -> bool:
+    """Try pngpaste first (fast, handles more formats), fall back to osascript."""
+    return _macos_pngpaste(dest) or _macos_osascript(dest)
+
+
+def _macos_pngpaste(dest: Path) -> bool:
+    """Use pngpaste (brew install pngpaste) — fastest, cleanest."""
+    try:
+        r = subprocess.run(
+            ["pngpaste", str(dest)],
+            capture_output=True, timeout=3,
+        )
+        if r.returncode == 0 and dest.exists() and dest.stat().st_size > 0:
+            return True
+    except FileNotFoundError:
+        pass  # pngpaste not installed
+    except Exception as e:
+        logger.debug("pngpaste failed: %s", e)
+    return False
+
+
+def _macos_osascript(dest: Path) -> bool:
+    """Use osascript to extract PNG data from clipboard (always available)."""
+    # First check if clipboard contains image data
+    try:
+        info = subprocess.run(
+            ["osascript", "-e", "clipboard info"],
+            capture_output=True, text=True, timeout=3,
+        )
+        has_image = "«class PNGf»" in info.stdout or "«class TIFF»" in info.stdout
+        if not has_image:
+            return False
+    except Exception:
+        return False
+
+    # Extract as PNG
+    script = (
+        'try\n'
+        '  set imgData to the clipboard as «class PNGf»\n'
+        f'  set f to open for access POSIX file "{dest}" with write permission\n'
+        '  write imgData to f\n'
+        '  close access f\n'
+        'on error\n'
+        '  return "fail"\n'
+        'end try\n'
+    )
+    try:
+        r = subprocess.run(
+            ["osascript", "-e", script],
+            capture_output=True, text=True, timeout=5,
+        )
+        if r.returncode == 0 and "fail" not in r.stdout and dest.exists() and dest.stat().st_size > 0:
+            return True
+    except Exception as e:
+        logger.debug("osascript clipboard extract failed: %s", e)
+    return False
+
+
+# ── Linux ────────────────────────────────────────────────────────────────
+
+def _linux_save(dest: Path) -> bool:
+    """Use xclip to extract clipboard image."""
+    # Check if clipboard has image content
+    try:
+        targets = subprocess.run(
+            ["xclip", "-selection", "clipboard", "-t", "TARGETS", "-o"],
+            capture_output=True, text=True, timeout=3,
+        )
+        if "image/png" not in targets.stdout:
+            return False
+    except FileNotFoundError:
+        logger.debug("xclip not installed — clipboard image paste unavailable")
+        return False
+    except Exception:
+        return False
+
+    # Extract PNG data
+    try:
+        with open(dest, "wb") as f:
+            subprocess.run(
+                ["xclip", "-selection", "clipboard", "-t", "image/png", "-o"],
+                stdout=f, stderr=subprocess.DEVNULL, timeout=5, check=True,
+            )
+        if dest.exists() and dest.stat().st_size > 0:
+            return True
+    except Exception as e:
+        logger.debug("xclip image extraction failed: %s", e)
+        dest.unlink(missing_ok=True)
+    return False