fix(cli): idx config prints JSON only when JSON is asked for

Auto output stays plain text when piped, as in 0.3.2, so scripts that
read $(idx config path) or $(idx config get KEY) keep working; -o json,
IDX_OUTPUT=json, or general.output = "json" print JSON. The config
goldens return to their 0.3.2 content.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V3syn2fN2LNo5pvTSqc6eh

crates/idx-cli/tests/golden/config-get.golden

@@ -1,4 +1,4 @@
 exit: 0
 --- stdout
-{"key":"general.provider","value":"msn"}
+msn
 --- stderr

crates/idx-cli/tests/golden/config-init.golden

@@ -1,4 +1,4 @@
 exit: 0
 --- stdout
-{"path":"<ROOT>/config/idx/config.toml"}
+<ROOT>/config/idx/config.toml
 --- stderr

crates/idx-cli/tests/golden/config-path.golden

@@ -1,4 +1,4 @@
 exit: 0
 --- stdout
-{"path":"<ROOT>/config/idx/config.toml"}
+<ROOT>/config/idx/config.toml
 --- stderr

crates/idx-cli/tests/golden/config-set-output-jsonl.golden

@@ -1,4 +1,4 @@
 exit: 0
 --- stdout
-{"key":"general.output","ok":true,"value":"jsonl"}
+ok
 --- stderr

crates/idx-cli/tests/golden/config-set.golden

@@ -1,4 +1,4 @@
 exit: 0
 --- stdout
-{"key":"general.provider","ok":true,"value":"yahoo"}
+ok
 --- stderr