fix: 13 skill review issues in /canvas

Critical:
- find precedence bug fixed: added \( -name '*.png' -o -name '*.jpg' \) grouping
  so -newer applies to BOTH extensions, not just .jpg
- Auto-positioning row y-detection: replaced broken right-half heuristic with
  min(n.y for all inside) — matches canvas-spec.md pseudocode
- WIKI.md canvas color table corrected: 1=red,2=orange,3=yellow,4=green,5=cyan,6=purple
  (was wrong mapping 1=blue etc from the graph CSS scheme, not canvas colors)

High:
- ID generation: full Unix timestamp (10 digits) instead of last-4-digits
  + added collision fallback (-2, -3 suffix)
- README: added /canvas add text and /canvas add pdf to commands table
- Aspect ratio: removed duplicate partial table from SKILL.md — now defers
  to canvas-spec.md as single source of truth
- new op: writes to wiki/overview.md not wiki/index.md (index has fixed schema)
- from banana: added explicit note that /banana is an optional external skill

Medium:
- add note: clarified 'type: file' vs 'type: link' — .md files use file nodes
- zone fallback y: changed from -140 (overlaps starter) to 280 (safe below all content)
- CLAUDE.md: added /save, /autoresearch, /canvas to Plugin Skills table

Low:
- allowed-tools: removed Grep (unused by any documented operation)
- Frontmatter: flattened description to single-line quoted string (fixes IDE
  YAML block scalar parsing diagnostic on lines 4-9)

README.md

@@ -83,7 +83,9 @@ Then scaffold the full wiki structure.
 | `/autoresearch [topic]` | Run the autonomous research loop: search, fetch, synthesize, file |
 | `/canvas` | Open or create the visual canvas, list zones and nodes |
 | `/canvas add image [path]` | Add an image (URL or local path) to the canvas with auto-layout |
-| `/canvas add note [page]` | Pin a wiki page as a card on the canvas |
+| `/canvas add text [content]` | Add a markdown text card to the canvas |
+| `/canvas add pdf [path]` | Add a PDF document as a rendered preview node |
+| `/canvas add note [page]` | Pin a wiki page as a linked card on the canvas |
 | `/canvas zone [name]` | Add a new labeled zone to organize visual content |
 | `/canvas from banana` | Capture recently generated images onto the canvas |
 | `lint the wiki` | Health check: orphans, dead links, gaps, suggestions |