security: implement pentest remediation (RATE-002, CLI-001)

RATE-002: Proactive rate limit cleanup when entries exceed threshold
- Add RATE_LIMIT_CLEANUP_THRESHOLD config (default 0.8)
- Trigger cleanup before hitting hard limit
- Prevents memory exhaustion under sustained load

CLI-001: Validate clipboard tool paths against trusted directories
- Add TRUSTED_CLIPBOARD_DIRS for Unix system paths
- Add TRUSTED_WINDOWS_PATTERNS for Windows validation
- Reject tools in user-writable locations (PATH hijack prevention)
- Use absolute paths in subprocess calls

fpaste

@@ -455,12 +455,65 @@ CLIPBOARD_WRITE_COMMANDS = [
     ("wl-copy", ["wl-copy"]),
 ]
 
+# CLI-001: Trusted directories for clipboard tools (system paths only)
+# Prevents command injection via malicious PATH manipulation
+TRUSTED_CLIPBOARD_DIRS = frozenset(
+    {
+        "/usr/bin",
+        "/usr/local/bin",
+        "/bin",
+        "/opt/homebrew/bin",  # macOS Homebrew
+        "/usr/X11/bin",
+        "/usr/X11R6/bin",
+    }
+)
+
+# Windows system directories (checked case-insensitively)
+TRUSTED_WINDOWS_PATTERNS = (
+    "\\windows\\",
+    "\\system32\\",
+    "\\syswow64\\",
+    "\\windowsapps\\",
+)
+
+
+def is_trusted_clipboard_path(path: str) -> bool:
+    """Check if clipboard tool path is in a trusted system directory.
+
+    CLI-001: Validates that resolved clipboard tool paths are in expected
+    system locations to prevent command injection via PATH manipulation.
+    """
+    if not path:
+        return False
+
+    resolved = Path(path).resolve()
+    parent = str(resolved.parent)
+
+    # Check Unix trusted directories
+    if parent in TRUSTED_CLIPBOARD_DIRS:
+        return True
+
+    # Check Windows paths (case-insensitive)
+    parent_lower = parent.lower()
+    for pattern in TRUSTED_WINDOWS_PATTERNS:
+        if pattern in parent_lower:
+            return True
+
+    # Also allow Windows Program Files paths
+    return "\\program files" in parent_lower
+
 
 def find_clipboard_command(commands: list[tuple[str, list[str]]]) -> list[str] | None:
-    """Find first available clipboard command."""
+    """Find first available clipboard command in trusted directories.
+
+    CLI-001: Validates that found commands are in trusted system directories
+    to prevent command injection via PATH manipulation.
+    """
     for tool_name, cmd in commands:
-        if shutil.which(tool_name):
-            return cmd
+        tool_path = shutil.which(tool_name)
+        if tool_path and is_trusted_clipboard_path(tool_path):
+            # Use absolute path for security
+            return [tool_path, *cmd[1:]]
     return None