Remove obsolete Compose version field

[aidankhogg]

Motivation

• Remove the top-level Compose file format declaration because modern Compose implementations follow the Compose Specification and the version field is now obsolete.

Description

• Deleted the version: "3.8" header from docker-compose.yml while keeping all other content unchanged, and updated docs/compose-brainstorm.md to record the decision to omit the top-level version field.

Testing

• Verified the updated docker-compose.yml parses with yaml.safe_load, ran python scripts/validate_supply_chain.py which passed, and ran git diff --check to ensure no whitespace/errors; docker compose was not executed because Docker is not available in the environment.

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Codex Task