Non-standard directory naming in several skills

Several skills use non-standard directory names instead of the canonical `scripts/`, `references/`, `assets/` pattern documented in the skill-creator guide:

| Skill | Current Directory | Suggested |
|-------|------------------|-----------|
| `algorithmic-art` | `templates/` | `assets/` (files used in output) |
| `canvas-design` | `canvas-fonts/` | `assets/fonts/` |
| `slack-gif-creator` | `core/` | `scripts/` (executable code) |
| `xlsx` | root-level `recalc.py` | `scripts/recalc.py` |

This isn't breaking, but inconsistent naming makes it harder for the skill-creator's validation tools to verify structure and for new skill authors to follow conventions.