feat: add model and budget configs to cortex_agent materialization

[Matts52]

Summary

Closes #19. Promotes models.orchestration and orchestration.budget from hand-written spec YAML to first-class config() options in specification mode, mirroring the existing web_search_tool, comment, and profile configs.

• New model config (string): sets models.orchestration in the spec YAML, e.g. model = 'claude-opus-4'
• New budget config: accepts a plain int as shorthand for {seconds: <n>}, or a dict with seconds/tokens keys; injects an orchestration.budget block into the spec YAML
• Adds cortex_agent_render_model_and_budget helper macro that prepends the YAML block before the user's spec body
• Emits compile-time warnings if the spec body already contains a conflicting top-level models: or orchestration: key, or if either config is used with raw_ddl=true
• Two new integration test models cover the dict form and integer shorthand

Test plan

• Run dbt run in integration_tests/ — agent_with_model_and_budget and agent_with_budget_shorthand should compile and execute without error
• Inspect target/compiled/ to confirm the YAML preamble (models:, orchestration:) appears correctly before the spec body
• Verify integer-shorthand budget produces only seconds: (no tokens: line)
• Confirm the raw_ddl=true warning now names model and budget

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