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/transcript.txt

@@ -0,0 +1,11 @@
+Iris: We could use one generic intake checklist for analytics pilots and hardware trials.
+
+Tom: I object to that. My position is that generic criteria will not work for these two types of work.
+
+Iris: Understood. Are you taking responsibility for rewriting the checklist?
+
+Tom: No. I am not taking that on, and I am not proposing an owner. I am only stating my position.
+
+Uma: Then we are not deciding the checklist today.
+
+Iris: Correct. No decision and no task for Tom.