feat(cli): agent-native output, stdin symbols, and exit-code classes

Phase 1 of docs/DESIGN.md:

- Output format resolves -o > IDX_OUTPUT > general.output > auto; auto
  is a table on a terminal and JSON when stdout is piped or captured.
  `config init` now writes output = "auto".
- JSON is pretty only on a terminal and compact otherwise. New `-o
  jsonl` prints one array item per line with the same key order.
- No ANSI color when stdout is not a terminal (direct owo-colors calls
  now go through output::style).
- `stocks quote -` and `stocks compare -` read symbols from stdin.
- Exit codes by class: 2 invalid input/config, 3 not found or cache
  miss, 4 upstream unavailable, 5 unsupported, 1 other.

Golden changes are limited to those documented kinds and were verified
mechanically: JSON goldens are semantically identical, tables differ
only by removed ANSI codes and the column padding comfy-table had
reserved for them (cell contents identical), exit codes match each
error's class, and help/completions only gain the new --output values.
17 new golden cases cover auto, jsonl, stdin, and the new errors.

tests/cli.rs now pins IDX_OUTPUT=table in its harness (its tests assert
table text and run piped) and parses discover JSON instead of matching
pretty-printed whitespace.

Live A/B against main: 16/16 stocks cases identical; ownership discover
got IDX 403 on both binaries, now exiting 4 instead of 1 as intended.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/golden/ownership-graph-dot.json.golden

@@ -1,58 +1,4 @@
 exit: 0
 --- stdout
-{
-  "nodes": [
-    {
-      "id": "entity:14",
-      "label": "DWIMURIA INVESTAMA ANDALAN",
-      "node_type": "entity"
-    },
-    {
-      "id": "entity:15",
-      "label": "CATURGUWIRATNA SUMAPALA",
-      "node_type": "entity"
-    },
-    {
-      "id": "entity:16",
-      "label": "TRICIPTA MANDHALA GUMILANG",
-      "node_type": "entity"
-    },
-    {
-      "id": "entity:17",
-      "label": "ANTHONI SALIM",
-      "node_type": "entity"
-    },
-    {
-      "id": "ticker:BBCA",
-      "label": "BBCA",
-      "node_type": "ticker"
-    }
-  ],
-  "edges": [
-    {
-      "from": "entity:17",
-      "to": "ticker:BBCA",
-      "percentage_bps": 115,
-      "source": "ksei"
-    },
-    {
-      "from": "entity:15",
-      "to": "ticker:BBCA",
-      "percentage_bps": 102,
-      "source": "ksei"
-    },
-    {
-      "from": "entity:14",
-      "to": "ticker:BBCA",
-      "percentage_bps": 5494,
-      "source": "ksei"
-    },
-    {
-      "from": "entity:16",
-      "to": "ticker:BBCA",
-      "percentage_bps": 107,
-      "source": "ksei"
-    }
-  ]
-}
+{"nodes":[{"id":"entity:14","label":"DWIMURIA INVESTAMA ANDALAN","node_type":"entity"},{"id":"entity:15","label":"CATURGUWIRATNA SUMAPALA","node_type":"entity"},{"id":"entity:16","label":"TRICIPTA MANDHALA GUMILANG","node_type":"entity"},{"id":"entity:17","label":"ANTHONI SALIM","node_type":"entity"},{"id":"ticker:BBCA","label":"BBCA","node_type":"ticker"}],"edges":[{"from":"entity:17","to":"ticker:BBCA","percentage_bps":115,"source":"ksei"},{"from":"entity:15","to":"ticker:BBCA","percentage_bps":102,"source":"ksei"},{"from":"entity:14","to":"ticker:BBCA","percentage_bps":5494,"source":"ksei"},{"from":"entity:16","to":"ticker:BBCA","percentage_bps":107,"source":"ksei"}]}
 --- stderr