feat: add extraction schema, sidebar nav, few-shot prompting, and prompt settings

Overhaul extraction pipeline with new TradeItem model, conversation flow,
and dedicated extraction endpoint. Add sidebar navigation with NavMenu
component and landing page. Introduce few-shot prompting service and
tests. Add prompt settings and email upload specs. Update OpenSpec
tooling with improved export-spec and extract-feature commands. Archive
completed changes and export full specs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

examples/extraction/few-shot/02/output.json

@@ -0,0 +1,12 @@
+{
+  "items": [
+    {
+      "valuedate": "15/03/2026",
+      "counterparty": "Deutsche Bank AG, London Branch",
+      "trade_id": 81200451,
+      "display_ccy": "USD",
+      "pv": 12750000,
+      "breakclause": "N"
+    }
+  ]
+}