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/stocks-insights.json.golden

@@ -1,16 +1,4 @@
 exit: 0
 --- stdout
-{
-  "id": "bn91jc",
-  "symbol": "BBCA.JK",
-  "summary": "Mixed analyst signals for Bank Central Asia Tbk PT: 1 positive, 1 negative, 1 neutral.",
-  "highlights": [
-    "P/E ratio: Earnings data unavailable, can't compare P/E ratio with peers",
-    "Analyst price target: Analysts forecast more than 20% upside for 12-month price target"
-  ],
-  "risks": [
-    "Quarterly Revenue YoY Growth: Revenue grew worse than peers"
-  ],
-  "last_updated": "2026-03-26T04:14:57.9197955Z"
-}
+{"id":"bn91jc","symbol":"BBCA.JK","summary":"Mixed analyst signals for Bank Central Asia Tbk PT: 1 positive, 1 negative, 1 neutral.","highlights":["P/E ratio: Earnings data unavailable, can't compare P/E ratio with peers","Analyst price target: Analysts forecast more than 20% upside for 12-month price target"],"risks":["Quarterly Revenue YoY Growth: Revenue grew worse than peers"],"last_updated":"2026-03-26T04:14:57.9197955Z"}
 --- stderr