docs: fix 6 contradictions from CONTRADICTIONS_REPORT_20260627 (C5/C6/C17/C19/C2)

Six fixes for the c11_python doc sync (chronology row 3):

- C5 (Result notation): Result[str, ErrorInfo] -> Result[str] at
  docs/guide_ai_client.md lines 452 + 469; also error_handling.md
  line 801 (historical deprecation section).
- C6 (RAGChunk schema): docs/guide_models.md lines 343-349 corrected
  to match src/rag_engine.py:19-25 (id, document, path, score, metadata).
- C17 (type_aliases.md table): rewrote alias table to reflect post-2026-06-25
  reality (Metadata is @dataclass(frozen=True, slots=True) with 36 fields;
  11 per-aggregate dataclasses listed with source locations; removed
  stale 'underlying type is dict[str, Any]' claim at line 73 + the
  'keep Metadata as dict[str, Any]' claim at line 81).
- C19 (OBLITERATE principle): added 'OBLITERATE Principle' section to
  error_handling.md after Migration Playbook; clarified in Hard Rules
  that argument types that may be None (caller choice) are NOT banned.
- C2 (audit script name): docs/AGENTS.md references updated to point
  to scripts/audit_optional_returns.py (the all-src/ successor to
  scripts/audit_optional_in_3_files.py).

Also: docs/reports/CONTRADICTIONS_REPORT_20260627.md — the contradictions
index that drives these fixes. Kept for reference.

C16 + C18 were already addressed in commit 770c2fdb (python.md §10
Documented Exceptions table + §17.10 audit inventory).

docs/guide_models.md

@@ -340,13 +340,13 @@ class RAGConfig:
     top_k: int = 5
     external_mcp_server: str | None = None
 
-@dataclass
+@dataclass(frozen=True)
 class RAGChunk:
-    text: str
-    source_path: str
-    start_line: int
-    end_line: int
-    embedding: list[float] = field(default_factory=list)
+    id:       str       = ""
+    document: str       = ""
+    path:     str       = ""
+    score:    float     = 0.0
+    metadata: Metadata  = field(default_factory=dict)
 
 @dataclass
 class RAGResult: