feat: add compose stack validation before container launch

[boffin-dmytro]

Summary

• Validate Docker Compose configuration syntax before launching containers
• Fail fast with clear error messages if compose files are invalid
• Prevents cryptic failures during container startup

Rationale

Compose file errors (syntax errors, missing variables, invalid service definitions) cause cryptic failures during container launch. Validating the merged compose stack upfront catches these issues early and provides actionable error messages.

Changes

• Run docker compose config --quiet to validate merged stack
• Check validation before launching containers in phase 11
• Provide clear error messages pointing to compose files and logs

Test Plan

• Verify validation passes with valid compose files
• Verify validation fails with syntax errors
• Verify validation fails with missing required variables
• Verify error messages are actionable

[boffin-dmytro]

Closing - will recreate against upstream repo