Support ${VAR} environment variable interpolation in JSON config files

[mcwalrus]

Closes #1429

Summary

Adds ${VAR} environment variable config interpolations.

• Resolves ${VAR} from os.environ at load time.
• Warns to stderr and leaves the placeholder unchanged if the variable is undefined.
• Non-string values, nested structures, and lists are all handled correctly.
• Applies to keys.json, aliases.json, and model_options.json.
• Existing configs without ${} are unaffected.

Test Coverage

13 tests covering:

• Basic ${VAR} resolution
• Undefined variables (warning + passthrough)
• Multiple variables in one string
• Plain $VAR (no braces) left unchanged
• Nested dict/list recursion
• Mixed types