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>

docs/DESIGN.md

@@ -1,6 +1,6 @@
 # System Design: idx-core and its frontends
 
-**Status:** Accepted 2026-10-14. Implementation on `refactor/idx-core`; nothing implemented yet.
+**Status:** Accepted 2026-10-14. Implementation on `refactor/idx-core`; phases 0-1 done.
 
 ## Goal
 
@@ -164,8 +164,8 @@ is another 20-30% smaller than pretty JSON.
   Already true.
 - No ANSI color when stdout is not a terminal. Today some table headers emit
   bold codes even when piped.
-- Commands that take symbols accept `-` to read them from stdin, one per line or
-  whitespace-separated:
+- `stocks quote` and `stocks compare` accept `-` to read symbols from stdin,
+  separated by whitespace or commas:
   `idx stocks screen --filter low-pe | jq -r '.[].symbol' | idx stocks compare -`
 - Exit codes by error class (today every error exits `1`):
 
@@ -175,7 +175,7 @@ is another 20-30% smaller than pretty JSON.
   | 1 | other failure | `IO`, `DATABASEERROR`, `PARSEERROR`, ... |
   | 2 | invalid usage or input | clap errors, `INVALIDINPUT`, `CONFIGERROR` |
   | 3 | not found or no data | `SYMBOLNOTFOUND`, `NOMARKETDATA`, `CACHEMISS` |
-  | 4 | upstream unavailable, retry later | `HTTP`, `RATELIMITED`, `PROVIDERUNAVAILABLE`, `OFFLINE` |
+  | 4 | upstream unavailable, retry later | `HTTP`, `RATELIMITED`, `PROVIDERUNAVAILABLE`, `OFFLINE`, `AUTHERROR` |
   | 5 | unsupported | `UNSUPPORTED` |
 
 - JSON keeps today's top-level shape: no `{data, meta}` wrapper, so `jq` paths