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

.github/plugin/marketplace.json

@@ -1024,6 +1024,12 @@
       "description": "Security frameworks, accessibility guidelines, performance optimization, and code quality best practices for building secure, maintainable, and high-performance applications.",
       "version": "1.0.0"
     },
+    {
+      "name": "signals-dashboard",
+      "source": "extensions/signals-dashboard",
+      "description": "Real-time agent coordination dashboard for The Workshop. Shows desk status, signal types (done, checkpoint, blocked, hands-up, partnership), intent text, outcome pairing with honesty gap, token usage, and stash/restore controls.",
+      "version": "0.1.0"
+    },
     {
       "name": "site-studio",
       "source": "extensions/site-studio",