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/README.md

@@ -0,0 +1,18 @@
+# position_explicit_objection
+
+Tests extraction of one explicit objection as a position.
+
+Scope:
+
+- Extract Tom's stated objection as exactly one position.
+- Do not create a todo for Tom.
+- Do not derive a decision from the objection.
+
+Exclusions:
+
+- This scenario does not test responsibility attribution for an open owner.
+- Responsibility attribution is covered by `responsibility_attribution_negative`.
+
+The transcript gives unique ground truth because Tom explicitly says "I object"
+and "My position is", then explicitly refuses responsibility. The group also
+states that no decision and no task for Tom were created.