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-history-msn.json.golden

@@ -1,29 +1,4 @@
 exit: 0
 --- stdout
-[
-  {
-    "date": "2026-01-14",
-    "open": 8000,
-    "high": 8000,
-    "low": 8000,
-    "close": 8000,
-    "volume": 0
-  },
-  {
-    "date": "2026-01-15",
-    "open": 8075,
-    "high": 8075,
-    "low": 8075,
-    "close": 8075,
-    "volume": 0
-  },
-  {
-    "date": "2026-01-16",
-    "open": 8025,
-    "high": 8025,
-    "low": 8025,
-    "close": 8025,
-    "volume": 0
-  }
-]
+[{"date":"2026-01-14","open":8000,"high":8000,"low":8000,"close":8000,"volume":0},{"date":"2026-01-15","open":8075,"high":8075,"low":8075,"close":8075,"volume":0},{"date":"2026-01-16","open":8025,"high":8025,"low":8025,"close":8025,"volume":0}]
 --- stderr