feat: add cds config command for persisted project defaults

[RonaldHensbergen]

Summary

Implements #383: a cds config subcommand family to manage persisted project-level defaults in .cds/config.json (or CDS_CONFIG_PATH), generalizing the existing single-purpose cds use command.

• cds config get <key> / set <key> <value> / unset <key> / list
• Supported keys: profile, environment, security.strict
• cds config set environment <name> validates the overlay exists for the configured default profile (reusing resolve_profile), mirroring the existing cds use profile validation
• cds config set security.strict true forces the production security rule class (infer_profile_class override) regardless of the profile's declared environment
• A saved environment default is applied whenever --environment is omitted, for all commands that already accept --environment (validate, plan, render, up, test, preflight, init, security); an explicit --environment flag always takes precedence, and rendering a saved Plan file still rejects an explicit --environment (a config-derived default is silently ignored there, since the overlay was already baked into the plan)
• cds use is unchanged / kept as the profile-only convenience command; both now share _read_config/_write_config
• README updated: new command table row + "Project defaults" section + precedence notes

Testing

• python -m unittest discover -s tests -p "test_*.py" -v — 561 tests, all pass
• make lint — passes

Resolves #383