TableSchema.name duplicates AGENTS_SCHEMA; collapse the redundancy

Every table literal carries a qualified name (`TableSchema("AGENTS.DBT_MODEL", ...)`), but every writer ignores the prefix and builds physical names as `f"{AGENTS_SCHEMA}.{table.base_name}"`. The prefix is a label with nothing validating it against the constant, so the label and physical reality can drift. They already did: the constant said `agents` while Snowflake stored `AGENTS` (#42).

Two coherent fixes, pick one:

1. Store base names only (`TableSchema("DBT_MODEL", ...)`) and derive the qualified name from the constant where a label is needed.
2. Make `name` authoritative: writers use `table.name` directly and drop the constant.

Option 1 keeps the single placement switch; option 2 makes each literal self-contained but scatters the schema name across eight files. Leaning option 1.

Tests assert on `table.name` as a label (`FakeDestination`), so either change ripples there mechanically.