Implements all spec-required output for `agents config get`:
Rich output:
- Rename panel title from 'Configuration Value' to 'Config' per spec
- Add 'Overridden' field to the Config panel
- Add 'Origin' panel showing File, Line, and Default fields
- Add 'Resolution Chain' panel (always shown, not just with --verbose)
- Add 'Winner' indicator to the Resolution Chain panel
- Use spec-required type strings (string/boolean/integer) instead of
Python type names (str/bool/int)
- Add '✓ OK Config read' confirmation message
JSON output:
- Wrap result in standard envelope (command, status, exit_code, data,
timing, messages)
- Add 'overridden' field to data
- Add 'origin' nested object (file, line, default)
- Add 'winner' nested object (source, level)
- Use human-readable source names (CLI flag, Env var, Config file,
Default) instead of internal enum values
- Add 'started' timestamp to timing
BDD:
- Add features/config_get_spec_output.feature with 20 scenarios
covering all Rich panels and JSON envelope fields
- Update existing tests to match new spec-compliant output
ISSUES CLOSED: #3423
