Own the SQLMesh start date here and move dbt to 1.12

[cnstlungu]

Why

Two things, both prompted by the dbt v2 release.

The start date. SQLMesh refuses to load a dbt project without a backfill start date, and it only looks for one in the models: block of dbt_project.yml. The shared datamart carried +start: Jan 1 2000 there purely for this stack — it reads as a stray to anyone working on the model, and the improvement plan had it down for deletion. dbt v2 settles it: +start is an unrecognised key and a hard parse error.

So the start date moves here, as model_defaults on sqlmesh_config, which is where a consumer-specific setting belongs.

dbt 1.10 is deprecated as of the v2 release. SQLMesh declares dbt-core<2, so 1.12 needs nothing else from us.

Merge order

This is safe to merge before the datamart drops +start: SQLMesh takes the config.py value whether or not dbt_project.yml still has one. It has to merge before DATAMART_REF moves to a tag without it.

Verification

Ran sqlmesh plan against the datamart branch that drops +start, on dbt-core 1.12.5 + dbt-duckdb 1.11.0:

• 23 models loaded, plan applied, all audits passed
• sources still resolve to READ_PARQUET(...) — the datamart branch declares external_location under both config and config.meta so v2 and SQLMesh each find it
• also verified with +start still present, which is the merge-order case above

duckdb stays at 1.4.4 so the warehouse remains readable by Superset.

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