feat(type_aliases): add per-aggregate dataclasses for metadata_promotion_20260624

TIER-2 READ AGENTS.md conductor/workflow.md conductor/edit_workflow.md conductor/tier2/githooks/forbidden-files.txt conductor/tracks/tier2_leak_prevention_20260620/spec.md conductor/code_styleguides/data_oriented_design.md conductor/code_styleguides/error_handling.md conductor/code_styleguides/type_aliases.md before Phase 0 Tasks 0.1, 0.2, 0.4.

Phase 0 of metadata_promotion_20260624. 11 NEW per-aggregate dataclasses added to src/type_aliases.py (CommsLogEntry, HistoryMessage, FileItem, ToolDefinition, SessionInsights, DiscussionSettings, CustomSlice, MMAUsageStats, ProviderPayload, UIPanelConfig, PathInfo) + RAGChunk added to src/rag_engine.py. Metadata: TypeAlias = dict[str, Any] preserved unchanged as the catch-all for collapsed codepaths. Each dataclass has paired to_dict()/from_dict() methods.

11 regression-guard test files created with 5-7 tests each (~70 tests total). All tests PASS.

The existing tests/test_type_aliases.py was updated to reflect the NEW design (CommsLogEntry etc. are now classes, not aliases to Metadata).

Conventions: 1-space indentation, CRLF preserved, no comments.

src/rag_engine.py

@@ -4,16 +4,34 @@ import json
 import os
 import sys
 
+from dataclasses import dataclass, field, fields as dc_fields
 from typing import List, Dict, Any, Optional
 
 from src import ai_client
 from src import models
 from src import mcp_client
 from src.result_types import ErrorInfo, ErrorKind, NilRAGState, Result
+from src.type_aliases import Metadata
 
 from src.file_cache import ASTParser
 
 
+@dataclass(frozen=True)
+class RAGChunk:
+ document: str = ""
+ path:     str = ""
+ score:    float = 0.0
+ metadata: Metadata = field(default_factory=dict)
+
+ def to_dict(self) -> Metadata:
+  return {f.name: getattr(self, f.name) for f in dc_fields(self)}
+
+ @classmethod
+ def from_dict(cls, data: Metadata) -> "RAGChunk":
+  valid = {f.name for f in dc_fields(cls)}
+  return cls(**{k: v for k, v in data.items() if k in valid})
+
+
 _SENTENCE_TRANSFORMERS = None
 _GOOGLE_GENAI          = None
 _CHROMADB              = None