feat(draw): wire the drawing card into the conversation — end to end (T-318)

The last wire: a DrawingMessage conversation item + the Claude-extension
subscriber on the `draw` channel (buildSvgDocument → inject), the
conversation renderer (DrawingCard), the fold/summary switches, the i18n
`drawing` label, and registering `clide draw` at boot (empty template
registry for now — primitive SVG works; d2/icon/compare/image handlers
plug in as they land). `clide draw --file doc.json` now renders a card in
the conversation. Render widget-tested; analyze + format clean across the
repo. Templates + per-object overlay remain; the CHANGELOG entry lands
with the user-complete feature.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

assets/i18n/en_us/builtin.claude.json

@@ -47,6 +47,7 @@
   "conversation.label.thinking": { "translation": "thinking" },
   "conversation.label.agentThinking": { "translation": "agent thinking" },
   "conversation.label.image": { "translation": "image" },
+  "conversation.label.drawing": { "translation": "drawing" },
   "conversation.label.agentRun": { "translation": "agent run" },
   "conversation.label.workflow": { "translation": "workflow" },
   "conversation.label.error": { "translation": "error" },

assets/i18n/nl_nl/builtin.claude.json

@@ -47,6 +47,7 @@
   "conversation.label.thinking": { "translation": "nadenken" },
   "conversation.label.agentThinking": { "translation": "agent denkt na" },
   "conversation.label.image": { "translation": "afbeelding" },
+  "conversation.label.drawing": { "translation": "tekening" },
   "conversation.label.agentRun": { "translation": "agent-uitvoering" },
   "conversation.label.workflow": { "translation": "workflow" },
   "conversation.label.error": { "translation": "fout" },