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/ownership-releases.json.golden

@@ -1,29 +1,4 @@
 exit: 0
 --- stdout
-[
-  {
-    "id": 1,
-    "source_url": null,
-    "sha256": "7c1a4c209b8ce5bde9112436a0c64d66b042915995b584d2bbf7ca7b15ce81e3",
-    "as_of_date": "2026-08-31",
-    "row_count": 17,
-    "imported_at": 0
-  },
-  {
-    "id": 3,
-    "source_url": null,
-    "sha256": "7b8da1d820f67062410c48988bb6c70fec2088c24961d481d1c0fce8b9372547",
-    "as_of_date": "2026-02-27",
-    "row_count": 18,
-    "imported_at": 0
-  },
-  {
-    "id": 2,
-    "source_url": null,
-    "sha256": "efb3d8fb19c9e0d803dc1b1734c8389356605d0f78d73db6729fb76d3d75753d",
-    "as_of_date": "2026-01-30",
-    "row_count": 18,
-    "imported_at": 0
-  }
-]
+[{"id":1,"source_url":null,"sha256":"7c1a4c209b8ce5bde9112436a0c64d66b042915995b584d2bbf7ca7b15ce81e3","as_of_date":"2026-08-31","row_count":17,"imported_at":0},{"id":3,"source_url":null,"sha256":"7b8da1d820f67062410c48988bb6c70fec2088c24961d481d1c0fce8b9372547","as_of_date":"2026-02-27","row_count":18,"imported_at":0},{"id":2,"source_url":null,"sha256":"efb3d8fb19c9e0d803dc1b1734c8389356605d0f78d73db6729fb76d3d75753d","as_of_date":"2026-01-30","row_count":18,"imported_at":0}]
 --- stderr