v0.2.15

Documentation sweep. Aligns README, docs/, the in-app wiki, the agent system prompt, and a couple of code paths with the canonical syntax that havn init already scaffolds.

No engine behaviour changes. Both @config / @depends_on / @assert and the legacy -- config: / -- depends_on: / -- assert: SQL-comment form still parse, so existing projects keep working.

What changed in the docs

• The transform example everywhere is now:

  @config materialized=table, schema=gold
  
  SELECT ...

  with a sentence below it pointing out that bronze.foo and silver.bar are picked up as dependencies automatically from FROM / JOIN clauses.

• @depends_on is documented as an optional override for the cases where the parser can't see a reference (a model name passed through a function or built up in a string).

• The @config table now lists unique_key, incremental_strategy, incremental_filter, and partition_by keys for incremental models.

• @description, @col, and @assert directives documented in the same place as @config.

What changed in code paths that emit syntax

• havn agent system prompt now teaches the agent the @-prefixed form and auto-extraction.
• The POST /api/models new-model scaffold writes @config materialized=..., schema=... instead of -- config: ....
• engine/notebook/conversion.py (notebook SQL cell -> model file) emits @config, @depends_on, @description in canonical form.

Tests updated to match the new emit. Full suite: 1245 passed, 72 skipped.