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/yahoo-quote.json.golden

@@ -1,20 +1,4 @@
 exit: 0
 --- stdout
-[
-  {
-    "symbol": "BBCA.JK",
-    "price": 9875,
-    "change": 117,
-    "change_pct": 1.1990161918425908,
-    "volume": 12300000,
-    "market_cap": 1215200000000000,
-    "week52_high": 10250,
-    "week52_low": 7800,
-    "week52_position": 0.8469387755102041,
-    "range_signal": "upper",
-    "prev_close": 9758,
-    "avg_volume": 10000000,
-    "as_of": null
-  }
-]
+[{"symbol":"BBCA.JK","price":9875,"change":117,"change_pct":1.1990161918425908,"volume":12300000,"market_cap":1215200000000000,"week52_high":10250,"week52_low":7800,"week52_position":0.8469387755102041,"range_signal":"upper","prev_close":9758,"avg_volume":10000000,"as_of":null}]
 --- stderr