Implement Meeting Context V1 and extraction improvements

Introduce Meeting Context V1 with YAML schema, validation and template.
Support optional --meeting-context during chunk extraction.
Inject authoritative Meeting Context into extraction prompts.
Record Meeting Context provenance in extraction output.
Activate todos.md in shared prompt assembly.
Strengthen responsibility attribution and decision/todo boundaries.
Add focused Gold scenarios and validation tests.
Update architecture and pipeline documentation.

tests/gold/responsibility_attribution_negative/expected.json

@@ -28,12 +28,6 @@
     }
   ],
   "questions": [],
-  "positions": [
-    {
-      "speaker": "Noah",
-      "position": "Noah says generic criteria will not work for different Marketing and product contexts.",
-      "evidence": "Noah: From Marketing, I can tell you that generic criteria will not work. A digital campaign and a physical product launch need different checks."
-    }
-  ],
+  "positions": [],
   "technical": []
 }