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-profile.json.golden

@@ -1,27 +1,4 @@
 exit: 0
 --- stdout
-{
-  "id": "feee16af-c94c-465d-a59d-a9727c22bd0e",
-  "symbol": "BBCA",
-  "short_name": "Bank Cntrl Asia",
-  "long_name": "PT Bank Central Asia Tbk",
-  "description": "PT Bank Central Asia Tbk (the Bank) is an Indonesia-based commercial bank.",
-  "sector": "Financials",
-  "industry": "Banking Services",
-  "website": "https://www.bca.co.id/",
-  "employees": 27937,
-  "address": "Menara BCA, Grand, Jl. MH Thamrin No. 1",
-  "city": "JAKARTA PUSAT",
-  "country": "Indonesia",
-  "phone": "+62 21 23588000",
-  "officers": [
-    {
-      "name": "Jahja Setiaatmadja",
-      "title": "President Director",
-      "age": 69,
-      "year_born": 1955,
-      "total_pay": 12000000000
-    }
-  ]
-}
+{"id":"feee16af-c94c-465d-a59d-a9727c22bd0e","symbol":"BBCA","short_name":"Bank Cntrl Asia","long_name":"PT Bank Central Asia Tbk","description":"PT Bank Central Asia Tbk (the Bank) is an Indonesia-based commercial bank.","sector":"Financials","industry":"Banking Services","website":"https://www.bca.co.id/","employees":27937,"address":"Menara BCA, Grand, Jl. MH Thamrin No. 1","city":"JAKARTA PUSAT","country":"Indonesia","phone":"+62 21 23588000","officers":[{"name":"Jahja Setiaatmadja","title":"President Director","age":69,"year_born":1955,"total_pay":12000000000}]}
 --- stderr