fix(config): emit section and section_value in JSON output for config subcommands (#2990)

`claw config model --output-format json` and all other section subcommands
(`env`, `hooks`, `plugins`) returned identical output with no section field
— the section arg was parsed but discarded (_section parameter).

Fix: render_config_json now:
- Passes section through to handler
- Looks up the section value via runtime_config.get(), converting the
  internal JsonValue to serde_json::Value via render()+parse
- Emits `section` (string) and `section_value` (JSON value or null)
  in the response envelope
- Returns ok:false + error for unsupported section tokens

Test: config_section_json_emits_section_and_value asserts:
- No section field when no section arg
- section + section_value fields present for all known sections
- ok:false + error for unknown section

Pinpoint: ROADMAP #126

rust/crates/rusty-claude-cli/tests/output_format_contract.rs

@@ -433,6 +433,44 @@ fn resumed_version_and_init_emit_structured_json_when_requested() {
     assert!(root.join("CLAUDE.md").exists());
 }
 
+#[test]
+fn config_section_json_emits_section_and_value() {
+    let root = unique_temp_dir("config-section-json");
+    fs::create_dir_all(&root).expect("temp dir should exist");
+
+    // Without a section: should return base envelope (no section field).
+    let base = assert_json_command(&root, &["--output-format", "json", "config"]);
+    assert_eq!(base["kind"], "config");
+    assert!(base["loaded_files"].is_number());
+    assert!(base["merged_keys"].is_number());
+    assert!(
+        base.get("section").is_none(),
+        "no section field without section arg"
+    );
+
+    // With a known section: should add section + section_value fields.
+    for section in &["model", "env", "hooks", "plugins"] {
+        let result = assert_json_command(&root, &["--output-format", "json", "config", section]);
+        assert_eq!(result["kind"], "config", "section={section}");
+        assert_eq!(
+            result["section"].as_str(),
+            Some(*section),
+            "section field must match requested section, got {result:?}"
+        );
+        assert!(
+            result.get("section_value").is_some(),
+            "section_value field must be present for section={section}"
+        );
+    }
+
+    // With an unsupported section: should return ok:false + error field.
+    let bad = assert_json_command(&root, &["--output-format", "json", "config", "unknown"]);
+    assert_eq!(bad["kind"], "config");
+    assert_eq!(bad["ok"], false);
+    assert!(bad["error"].as_str().is_some());
+    assert!(bad["section"].as_str().is_some());
+}
+
 fn assert_json_command(current_dir: &Path, args: &[&str]) -> Value {
     assert_json_command_with_env(current_dir, args, &[])
 }