Integrate new instruction files with developer agent

[begelundmuller]

Changes:

• Changes the developer_agent to use the improved instruction files from Instructions for developer agents #8615
• Adds a generic develop_file sub-agent and removed the previous develop_model and develop_metrics_view sub-agents
• Injects context about the default OLAP connector (its name, driver, and read/readwrite mode)
• Exposes the introspection tools over MCP so Cursor/Claude can use them (tools: project_status, query_sql, list_tables, show_table, list_buckets, list_bucket_objects – see this PR: New tools for the developer agent #8600)
• Updates default OpenAI model to GPT 5.2
• Added four new evals (try these in Rill Developer!):
  • Clickhouse has a public playground database on play.clickhouse.com:9440 (username 'play' and no password). Can you set it up as the OLAP connector and develop a dashboard based on one of its tables?
  • I have some CSV files in S3. Can you create a connector for S3 and a DuckDB model that loads the data at s3://integration-test.rilldata.com/glob_test/y=*/*.csv? I've already added environment variables for S3 access.
  • I have some data in S3. Can you tell me what buckets are available? And also show me a little preview of what files are available at s3://integration-test.rilldata.com/glob_test/?
  • This metrics view has an error. Can you fix it?

Closes https://linear.app/rilldata/issue/PLAT-378/integrate-new-unified-instructions-into-the-developer-agent

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!