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.

prompts/todos.md

@@ -0,0 +1,16 @@
+Action-item responsibility rule:
+
+A named responsible person may be extracted only when the transcript contains
+clear evidence that the person was explicitly assigned the task, explicitly
+volunteered, or explicitly accepted/confirmed the task.
+
+The following are not assignments: addressing a person while discussing a
+topic; saying that a person or department will have different criteria;
+requesting input from a department; stating expertise, competence, or
+organizational role; suggestions, expectations, objections, or preferences;
+Meeting Context role or department metadata by itself.
+
+When work is clearly needed but no person accepted it, keep the action item
+without a responsible person if the schema allows it; otherwise do not create a
+named assignment. Meeting Context may validate identity, role, and attendance,
+but never establishes responsibility.