sessions-graph: extract each session under its user's adopted ontology version

[antejavor]

Stacked on #444, which is stacked on #441. Merge order: #441 → #444 → this. Retarget each one to main after its base merges, before merging it.

Build step 2 of map #431, "An ontology that learns from the user's sessions". This PR adds the version chain and the user-supplied schema path. sessions-graph derive, the step that produces learned versions, comes next.

What

• Versions in the graph (decided in Grill: how a derivation run aggregates into the previous model #434).
  • One (:OntologyVersion {user_id, version, status, source, derive, model, pinned, source_hash}) node per version, with the model stored as JSON. NEXT chains the versions. (:User)-[:ADOPTED]-> moves to the new version in a single query, so readers never see two adopted versions or none.
  • A uniqueness constraint covers (user_id, version).
  • A user with no stored version is on version 0, hygm.default_model(), which is never stored.
• Reconcile uses the adopted version. reconcile_session extracts with GLiNER2 over the session user's adopted model, keeping one backend per distinct model, all sharing one loaded checkpoint. It records the version used as Session.ontology_version. An explicit extraction_backend still overrides this; ontology_version is then left null.
• Supplied schemas (decided in Grill: the default ontology and a user-supplied one #436). supply() turns a schema into the user's next version, with its types pinned:
  • derive="extend" adds the fixed core, and carries forward types derivation learned on the current version, unless one clashes with a supplied label.
  • derive="off" uses the schema exactly as given.
  • Supplied schemas go through validate_model() only.
• CLI: sessions-graph ontology load --file … [--user] [--derive extend|off] and sessions-graph ontology show [--user]. --user defaults to identity.user_id.
• Config file: a new [ontology] section with path and derive, set via agent-context-graph config set ontology.path|ontology.derive. It survives config set and bootstrap rewrites, the same way [recall] does.
  • Each sessions-graph reconcile applies the file to the configured user when its content hash or derive mode has changed.
  • A file that fails validation is reported, and the current version is kept.
• hygm: model_from_mapping / model_to_mapping, split out of ManualStrategy, so a version's JSON goes through the same validation as a YAML file. Error messages for files are unchanged.

One deviation from the decision

#436 wrote the config section as [sessions_graph.ontology]. Here it is [ontology], to match the file's existing flat sections ([recall], [reconcile]). It's easy to rename if you'd rather keep the decision's spelling.

Tests

All run against a real Memgraph:

• test_e2e_ontology.py (10 tests):
  • a user with no version is on the default;
  • extend adds the core and pins the schema's types; off uses it as given;
  • new versions supersede and chain after the last;
  • an invalid schema writes nothing;
  • sync adopts only on a content or mode change;
  • learned types carry forward unless they clash;
  • the CLI load/show and its no-user error;
  • reconcile applies the configured file and survives a broken one.
• Reconcile: a session is extracted under the user's supplied version and records ontology_version = 1.
• Config: [ontology] round-trips through config set, survives bootstrap, and rejects an invalid derive value.

Suite	Result
sessions-graph	106 passed
agent-context-graph	165 passed
hygm	43 passed
unstructured2graph	145 passed
context-graph-eval	187 passed

ruff and ty check . are clean. agent-context-graph failed 3 tests once and then passed 9 runs in a row; I couldn't reproduce the failure.

[antejavor]

Added after this description: hygm.model_from_mapping(..., validate=False). It reads back a store of types that is never extracted against: a derived version's pool of retired relations can name types the pool doesn't hold. Found by a real learned-ontology BEAM run.