fix: load skills from external_dirs in WebUI skills page

[529349029]

Problem

The WebUI skills page (/skills) only scans ~/.hermes/skills/ for skills, completely ignoring skills.external_dirs from config.yaml. Skills in external directories (e.g., ~/commonSkills/) are invisible in the UI.

Fix

• getAllSkillsDirs(): reads skills.external_dirs from config and expands ~ to home directory
• list(): scans all directories, merges categories, deduplicates skills by name
• listFiles(): searches all directories for skill files
• readFile_(): searches all directories when reading skill content, with path traversal protection per directory

Testing

• Skills from ~/.hermes/skills/ still load as before
• Skills from external dirs now appear in the listing
• Duplicate skill names from different sources are merged (first source wins)
• Error handling: missing/denied directories are silently skipped