fix(cli): add missing Origin panel, Overridden field, Winner indicator, and JSON envelope in config get output

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

features/steps/config_cli_steps.py

@@ -143,9 +143,11 @@ def step_config_get_contains_key(context: Context, key: str) -> None:
 @then("the config get output should show the resolution chain")
 def step_config_get_resolution_chain(context: Context) -> None:
     output = context.result.output
-    assert "Resolution chain" in output or "resolution_chain" in output, (
-        f"Expected resolution chain in output: {output}"
-    )
+    assert (
+        "Resolution Chain" in output
+        or "Resolution chain" in output
+        or "resolution_chain" in output
+    ), f"Expected resolution chain in output: {output}"
 
 
 @then("the config get should fail with unknown key error")