feat(context): add ContextGraph Markdown round-trip

docs/guides/context-graphs.md

@@ -436,6 +436,29 @@ d = graph.to_dict()
 # d["statistics"] → {"node_count": int, "edge_count": int}
 ```
 
+For a human-editable, version-control-friendly representation, save a Markdown
+directory instead:
+
+```python
+graph.save_to_file("context_graph/", format="markdown")
+
+restored = ContextGraph(advanced_analytics=True)
+restored.load_from_file("context_graph/", format="markdown")
+```
+
+The directory contains a versioned `graph.md` manifest for graph identity,
+relationships, and cross-graph link descriptors, plus one file per node under
+`nodes/`. A node's content is its Markdown body; its ID, type, properties,
+metadata, and temporal validity are YAML frontmatter. Node, edge, family, graph,
+and cross-graph link IDs are preserved across round trips.
+
+Markdown loading uses replacement semantics, like `from_dict()`: it parses and
+validates the complete directory before replacing the current graph. Invalid YAML,
+duplicate IDs, dangling edge endpoints, unsupported versions, and unsafe symbolic
+links fail without partially mutating the graph. Re-exporting to an existing managed
+directory atomically replaces it, removing stale node files. To avoid accidental data
+loss, a non-empty directory without the ContextGraph manifest is never replaced.
+
 If the graph had cross-graph links created with `link_graph()`, call `resolve_links()` after loading to restore live navigation — object references cannot be serialized, so they must be reconnected manually:
 
 ```python

docs/reference/context.md

@@ -435,8 +435,8 @@ print("Nodes: {}, Edges: {}".format(stats["node_count"], stats["edge_count"]))
 | `query(query, skip, limit)` | `List[Dict]` | Full-text search over node content |
 | `stats()` | `Dict` | Node/edge counts, type breakdowns, graph density |
 | `density()` | `float` | Graph density score |
-| `save_to_file(path)` | `None` | Persist graph to JSON |
-| `load_from_file(path)` | `None` | Load graph from JSON |
+| `save_to_file(path, format="json")` | `None` | Persist graph as JSON or a Markdown directory |
+| `load_from_file(path, format="json")` | `None` | Replace graph state from JSON or a Markdown directory |
 | `build_from_conversations(conversations, link_entities)` | `Dict` | Build graph from conversation data |
 | `link_graph(other_graph, source_node_id, target_node_id, link_type)` | `str` | Create cross-graph navigation link; returns `link_id` |
 | `navigate_to(link_id)` | `Tuple` | Follow a cross-graph link to `(target_graph, target_node_id)` |