Preserve model and effort during managed updates

[protonspy]

Summary

• Preserve model and effort frontmatter overrides when csdd update refreshes managed agents and skills.
• Treat those overrides as local execution metadata so they do not create conflicts or .old backups by themselves.
• Add regression coverage for managed agents, managed skills, and template refreshes carrying overrides forward.

Tests

• env GOCACHE=/tmp/go-cache go test ./internal/cli
• pre-push: go test -race -coverprofile=coverage.out ./...