Feat: `bq-agent-sdk ontology-build --skip-property-graph` to populate base tables only

# Feat: `bq-agent-sdk ontology-build --skip-property-graph` to populate base tables only

## Goal

Add an opt-in flag on the `ontology-build` CLI that runs phases 1–4 (load spec → extract → create tables → materialize) and stops before phase 5 (`CREATE OR REPLACE PROPERTY GRAPH`). Lets users with a pre-defined BigQuery property graph populate its base tables from BQ AA traces without overwriting their graph DDL on every run.

## Motivation

`build_ontology_graph(...)` ([`ontology_orchestrator.py:291`](https://github.com/GoogleCloudPlatform/BigQuery-Agent-Analytics-SDK/blob/main/src/bigquery_agent_analytics/ontology_orchestrator.py#L291)) runs five phases in sequence:

1. Load spec.
2. Extract `ExtractedGraph` via `AI.GENERATE` on `agent_events`.
3. `OntologyMaterializer.create_tables()` — `CREATE TABLE IF NOT EXISTS` per entity/relationship table. Idempotent against pre-existing tables.
4. `OntologyMaterializer.materialize(...)` — staging-table → DELETE-by-session → INSERT FROM staging. Non-destructive, schema-aware via `bq_client.get_table(...).schema`.
5. `OntologyPropertyGraphCompiler.compile_property_graph_ddl(...)` then execute → **`CREATE OR REPLACE PROPERTY GRAPH`** ([`ontology_property_graph.py:307`](https://github.com/GoogleCloudPlatform/BigQuery-Agent-Analytics-SDK/blob/main/src/bigquery_agent_analytics/ontology_property_graph.py#L307)).

Phase 5 is destructive against a user-defined property graph. A user with their own `CREATE PROPERTY GRAPH` DDL — managed by Terraform, dbt, or hand-authored to express graph-object features the SDK doesn't generate yet — gets it overwritten on every `ontology-build` run.

The clean workaround today is dropping into Python and calling `OntologyGraphManager.extract_graph(...)` + `OntologyMaterializer.materialize(...)` directly, bypassing the orchestrator. That works but loses the CLI surface (`--ontology`, `--binding`, `--session-ids`, format output, error handling).

## Proposed change

One new flag on the `ontology-build` command in [`cli.py:1200`](https://github.com/GoogleCloudPlatform/BigQuery-Agent-Analytics-SDK/blob/main/src/bigquery_agent_analytics/cli.py#L1200):

```
bq-agent-sdk ontology-build \
--ontology my.ontology.yaml \
--binding my-bq-prod.binding.yaml \
--session-ids s1,s2,s3 \
--project-id my-project \
--dataset-id my-dataset \
--skip-property-graph
```

When `--skip-property-graph` is set:

- Phase 5 is not invoked.
- The result dict reports `property_graph_created: False` with `skipped_reason: "user_requested"` (distinct from today's `False` which means *attempted-and-failed*).
- **The CLI's printed/JSON output also exposes a disambiguating field.** Today's CLI output (the curated dict around [`cli.py:1266–1274`](https://github.com/GoogleCloudPlatform/BigQuery-Agent-Analytics-SDK/blob/main/src/bigquery_agent_analytics/cli.py#L1266)) only includes `property_graph_created`. Add a `property_graph_status` field with one of `"created"`, `"failed"`, or `"skipped:user_requested"`. Without this, JSON consumers see `property_graph_created: false` with exit 0 and no signal explaining why — locally consistent with the result dict but user-visibly ambiguous.
- The CLI exit-1 branch at [`cli.py:1277–1284`](https://github.com/GoogleCloudPlatform/BigQuery-Agent-Analytics-SDK/blob/main/src/bigquery_agent_analytics/cli.py#L1277) (which today raises `typer.Exit(code=1)` whenever `property_graph_created` is `False`) must be updated to short-circuit when `result.get("skipped_reason") == "user_requested"`. In that case, exit 0 with no error message. The "Property Graph creation failed" message stays for the *attempted-and-failed* branch.

The default stays `False` to preserve current behavior.

## Implementation sketch

`build_ontology_graph` gains a `skip_property_graph: bool = False` parameter. The phase-5 block becomes:

```python
if skip_property_graph:
property_graph_created = False
skipped_reason = "user_requested"
else:
# existing CREATE OR REPLACE PROPERTY GRAPH path
...
```

The CLI threads the flag through. Output dict gains `skipped_reason` only when populated.

## Acceptance criteria

- [ ] `bq-agent-sdk ontology-build --skip-property-graph ...` populates entity/relationship tables and exits 0 without invoking `CREATE OR REPLACE PROPERTY GRAPH`.
- [ ] The CLI exit-handling branch at [`cli.py:1277`](https://github.com/GoogleCloudPlatform/BigQuery-Agent-Analytics-SDK/blob/main/src/bigquery_agent_analytics/cli.py#L1277) is updated: when `property_graph_created` is `False` *and* `skipped_reason == "user_requested"`, the CLI exits 0 with no error printed. When `property_graph_created` is `False` for any other reason, the existing exit-1-with-error-message behavior is preserved.
- [ ] The CLI's curated output dict (around [`cli.py:1266–1274`](https://github.com/GoogleCloudPlatform/BigQuery-Agent-Analytics-SDK/blob/main/src/bigquery_agent_analytics/cli.py#L1266)) gains a `property_graph_status` field with values `"created"`, `"failed"`, or `"skipped:user_requested"` — both `text` and `json` formats expose it. Test asserts JSON consumers can distinguish skipped from failed without reading stderr.
- [ ] Without the flag, behavior is unchanged (regression test asserts exit 1 + the existing error message when phase 5 fails after successful materialization).
- [ ] A unit test asserts `OntologyPropertyGraphCompiler` is not constructed when the flag is set.
- [ ] An integration test (gated on `RUN_LIVE_BIGQUERY_TESTS=1`, matching the existing ontology integration test pattern at [`tests/test_integration_ontology_binding.py:44`](https://github.com/GoogleCloudPlatform/BigQuery-Agent-Analytics-SDK/blob/main/tests/test_integration_ontology_binding.py#L44)) creates a pre-existing property graph, runs `ontology-build --skip-property-graph` against pre-existing base tables, and verifies the user's graph definition is unchanged after the run.
- [ ] `docs/ontology/` (or wherever the orchestrator is documented) explains the flag and the use case.

## Out of scope

- Validating that the user's pre-existing property graph is consistent with the ontology+binding the SDK is materializing into. That is a separate pre-flight concern — see the companion issue for binding-vs-physical-schema validation.
- Skipping phase 3 (`create_tables`). It's already a no-op against pre-existing tables via `CREATE TABLE IF NOT EXISTS` ([`ontology_materializer.py:207, 210, 221, 241`](https://github.com/GoogleCloudPlatform/BigQuery-Agent-Analytics-SDK/blob/main/src/bigquery_agent_analytics/ontology_materializer.py#L207)). No flag needed.
- Auto-deriving an ontology+binding from a user's existing property graph DDL.

## Related

- #76 — extracted-graph validator (different concern; runs *after* extraction, *before* materialization).
- Companion issue: pre-flight validator that bound BQ tables physically contain the columns the binding declares with compatible types.

## Effort

~0.5 eng-day. CLI threading + one orchestrator branch + tests.