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-concentration.json.golden

@@ -1,45 +1,4 @@
 exit: 0
 --- stdout
-[
-  [
-    "AALI",
-    {
-      "top1_bps": 7968,
-      "top3_bps": 7968,
-      "hhi": 6348,
-      "free_float_bps": 2032,
-      "holder_count": 1
-    }
-  ],
-  [
-    "BBCA",
-    {
-      "top1_bps": 5494,
-      "top3_bps": 5716,
-      "hhi": 3021,
-      "free_float_bps": 4182,
-      "holder_count": 4
-    }
-  ],
-  [
-    "AADI",
-    {
-      "top1_bps": 4110,
-      "top3_bps": 6230,
-      "hhi": 2009,
-      "free_float_bps": 2030,
-      "holder_count": 10
-    }
-  ],
-  [
-    "ABBA",
-    {
-      "top1_bps": 283,
-      "top3_bps": 418,
-      "hhi": 9,
-      "free_float_bps": 9582,
-      "holder_count": 2
-    }
-  ]
-]
+[["AALI",{"top1_bps":7968,"top3_bps":7968,"hhi":6348,"free_float_bps":2032,"holder_count":1}],["BBCA",{"top1_bps":5494,"top3_bps":5716,"hhi":3021,"free_float_bps":4182,"holder_count":4}],["AADI",{"top1_bps":4110,"top3_bps":6230,"hhi":2009,"free_float_bps":2030,"holder_count":10}],["ABBA",{"top1_bps":283,"top3_bps":418,"hhi":9,"free_float_bps":9582,"holder_count":2}]]
 --- stderr