refactor(skills): absolute XDG paths for tangling

- Updated all 23 skill org files to use absolute path
- Tangle now outputs directly to ~/.local/share/opencortex/skills/
- Removed  env var (org-babel doesn't expand it)

skills/org-skill-credentials-vault.org

@@ -1,7 +1,7 @@
 #+TITLE: SKILL: Credentials Vault (org-skill-credentials-vault.org)
 #+AUTHOR: Agent
 #+FILETAGS: :system:security:vault:
-#+PROPERTY: header-args:lisp :tangle $OC_DATA_DIR/skills/org-skill-credentials-vault.lisp
+#+PROPERTY: header-args:lisp :tangle /home/user/.local/share/opencortex/skills/org-skill-credentials-vault.lisp
 
 * Overview
 The *Credentials Vault* provides secure in-memory storage for sensitive API keys and session tokens.