v1.0.2 — Block symlink information disclosure

Fixes symlink-based information disclosure vulnerability.

An attacker could create a symlink (e.g. ln -s ~/.ssh/id_rsa secrets.pdf) and instruct the agent to print it, leaking sensitive files to physical printer output.

Fix: _validate_file_path() now rejects symlinks entirely. File extension validation runs on both the literal path and the resolved target.