feat: add signals-dashboard canvas extension

The dashboard is the centerpiece — real-time agent coordination
view showing desk status, signal types, intent text, outcome
pairing with honesty gap, token usage, and stash/restore controls.

Includes all updates from the-workshop PRs #3-#7:
- Empty state guidance for new users
- Token usage display per desk
- TA partnership signals + Cairn awareness
- Intent-as-text (execution signals use descriptive text)
- Outcome signal pairing with honesty gap calibration
- Open desk button
- Subtype labels (done/checkpoint/blocked/hands-up)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 26faf13e-639c-4a21-ac05-c0dc2bff7c62

plugins/the-workshop/.github/plugin/plugin.json

@@ -24,5 +24,10 @@
     "./skills/desk-open/",
     "./skills/signal-write/",
     "./skills/workshop-create/"
-  ]
+  ],
+  "x-awesome-copilot": {
+    "extensions": [
+      "./extensions/signals-dashboard/"
+    ]
+  }
 }