feat(harness): authority, turn contract, tools, clean_agent_preview

Chat/agent harness work: request-authority model, turn contract, tool
policy/parsing/execution, clean_agent_preview streaming/thinking, ask_user
persistence, session probes, and scheduler.

Source: vendor/photocraft (ad1cff44) harness files, 3-way merged onto
upstream/dev (a8c147b2). Harness core files were independently developed
in both lineages; resolution policy documented in pr_work/C_conflicts.md
(vendor kept where ahead, upstream kept where vendor is behind; full delta
preserved in pr_work/C_upstream_delta.patch).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/agent_tools/document_tools.py

@@ -496,6 +496,7 @@ def _create_pdf_text_derivative(db, *, source_doc, content: str, owner: Optional
         "content": clean,
         "version": 1,
         "source_doc_id": getattr(source_doc, "id", None),
+        "exit_code": 0,
     }
 
 
@@ -618,6 +619,7 @@ class CreateDocumentTool:
                 "language": language,
                 "content": content,
                 "version": 1,
+                "exit_code": 0,
             }
         except Exception as e:
             db.rollback()
@@ -708,6 +710,7 @@ class UpdateDocumentTool:
                 "language": doc.language,
                 "content": new_content,
                 "version": new_ver,
+                "exit_code": 0,
             }
         except Exception as e:
             db.rollback()
@@ -863,6 +866,7 @@ class EditDocumentTool:
                         "version": new_ver,
                         "applied": applied,
                         "skipped": skipped,
+                        "exit_code": 0,
                     }
                 return {"error": "No edits applied — FIND text cannot be blank"}
 
@@ -968,6 +972,7 @@ class EditDocumentTool:
                 "version": new_ver,
                 "applied": applied,
                 "skipped": skipped,
+                "exit_code": 0,
             }
             if partial_edits:
                 result.update({
@@ -1059,6 +1064,7 @@ class SuggestDocumentTool:
                 "partial": bool(invalid),
                 "rejected": len(invalid),
                 "invalid_suggestions": invalid,
+                "exit_code": 0,
             }
         finally:
             db.close()