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/position_explicit_objection/expected.json

@@ -0,0 +1,14 @@
+{
+  "facts": [],
+  "decisions": [],
+  "todos": [],
+  "questions": [],
+  "positions": [
+    {
+      "speaker": "Tom",
+      "position": "Tom objects to using one generic intake checklist because generic criteria will not work for analytics pilots and hardware trials.",
+      "evidence": "Tom: I object to that. My position is that generic criteria will not work for these two types of work."
+    }
+  ],
+  "technical": []
+}