feat: /canvas command — visual layer of the wiki

New files:
  commands/canvas.md              — slash command with full trigger table
  skills/canvas/SKILL.md          — complete skill: open, new, add image/text/pdf/note,
                                    zone, list, from-banana operations
  skills/canvas/references/canvas-spec.md — Obsidian canvas JSON spec:
                                    coordinate system, all node types, color table,
                                    image sizing by aspect ratio, auto-positioning
                                    pseudocode, common mistakes, full example

Auto-positioning algorithm (bbox math):
  - Finds rightmost node in target zone
  - Places next node at rightmost_x + 40, same row y
  - If overflow: wraps to new row (max_y_in_zone + 20)
  - Falls back to below all content if no zone found

Banana integration:
  - /canvas from banana: checks .recent-images.txt first, then
    filesystem find -newer 10min, presents list for confirmation
  - After any /banana run: suggests /canvas from banana

Updated:
  skills/wiki/SKILL.md        — added /canvas to routing table
  skills/wiki/references/plugins.md — added Calendar + Thino with
                                      pre-installed note + manual install fallback
  README.md                   — /canvas in commands table + file structure

skills/wiki/SKILL.md

@@ -107,6 +107,7 @@ Route to the correct operation based on what the user says:
 | "lint", "health check", "clean up" | LINT | `wiki-lint` |
 | "save this", "file this", "/save" | SAVE | `save` |
 | "/autoresearch [topic]", "research [topic]" | AUTORESEARCH | `autoresearch` |
+| "/canvas", "add to canvas", "open canvas" | CANVAS | `canvas` |
 
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