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.

src/meeting_lab/llm/prompts.py

@@ -30,12 +30,14 @@ def build_extraction_prompt(
     source_name: str,
     transcript: str,
     output_schema: dict[str, Any],
+    meeting_context: str | None = None,
     task_prompt_names: Iterable[str] = ("decisions.md",),
     prompts_dir: Path = PROMPTS_DIR,
 ) -> str:
     schema_text = json.dumps(output_schema, ensure_ascii=False, indent=2)
     prompt_parts = [
         load_prompt("common.md", prompts_dir),
+        meeting_context,
         *load_existing_prompts(task_prompt_names, prompts_dir),
         f"""Quelldatei:
 {source_name}