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-fundamental.json.golden

@@ -1,41 +1,4 @@
 exit: 0
 --- stdout
-{
-  "symbol": "BBCA.JK",
-  "growth": {
-    "revenue_growth": 0.118,
-    "earnings_growth": 0.121,
-    "revenue_growth_pct": 11.8,
-    "earnings_growth_pct": 12.1,
-    "revenue_signal": "moderate",
-    "earnings_signal": "moderate",
-    "overall_signal": "growing"
-  },
-  "valuation": {
-    "pe_trailing": 25.4,
-    "pe_forward": 23.1,
-    "pb": 4.6,
-    "roe": 0.202,
-    "roe_pct": 20.2,
-    "net_margin": 0.385,
-    "net_margin_pct": 38.5,
-    "ev_ebitda": 21.28,
-    "pe_signal": "premium",
-    "pb_signal": "expensive",
-    "roe_signal": "excellent",
-    "margin_signal": "excellent",
-    "ev_ebitda_signal": "expensive",
-    "overall_signal": "expensive"
-  },
-  "risk": {
-    "debt_to_equity": 18.5,
-    "current_ratio": 1.21,
-    "roa": 0.038,
-    "roa_pct": 3.8,
-    "de_signal": "conservative",
-    "current_ratio_signal": "tight",
-    "overall_signal": "moderate risk"
-  },
-  "overall_signal": "mixed"
-}
+{"symbol":"BBCA.JK","growth":{"revenue_growth":0.118,"earnings_growth":0.121,"revenue_growth_pct":11.8,"earnings_growth_pct":12.1,"revenue_signal":"moderate","earnings_signal":"moderate","overall_signal":"growing"},"valuation":{"pe_trailing":25.4,"pe_forward":23.1,"pb":4.6,"roe":0.202,"roe_pct":20.2,"net_margin":0.385,"net_margin_pct":38.5,"ev_ebitda":21.28,"pe_signal":"premium","pb_signal":"expensive","roe_signal":"excellent","margin_signal":"excellent","ev_ebitda_signal":"expensive","overall_signal":"expensive"},"risk":{"debt_to_equity":18.5,"current_ratio":1.21,"roa":0.038,"roa_pct":3.8,"de_signal":"conservative","current_ratio_signal":"tight","overall_signal":"moderate risk"},"overall_signal":"mixed"}
 --- stderr