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/cases.toml

@@ -1,9 +1,11 @@
 # Golden output cases for tests/golden.rs.
 #
 # Each case runs once per entry in `formats` (default: table and json) with
-# `-o <format>` prepended; `formats = []` runs the args as given. Every run uses
-# the mock provider (IDX_USE_MOCK_PROVIDER=1) and an isolated home directory.
-# `ownership = true` seeds the fixture DB; `before` runs setup commands first.
+# `-o <format>` prepended; `formats = []` runs the args as given, which means
+# the auto format: stdout is a pipe here, so auto resolves to compact JSON.
+# Every run uses the mock provider (IDX_USE_MOCK_PROVIDER=1) and an isolated
+# home directory. `ownership = true` seeds the fixture DB; `before` runs setup
+# commands first; `stdin` is piped to the command.
 #
 # `idx version` is excluded on purpose: its output changes with every release.
 
@@ -62,6 +64,22 @@ name = "config-set"
 args = ["config", "set", "general.provider", "yahoo"]
 formats = []
 
+[[case]]
+name = "config-set-output-jsonl"
+args = ["config", "set", "general.output", "jsonl"]
+formats = []
+
+[[case]]
+name = "config-set-output-invalid"
+args = ["config", "set", "general.output", "yaml"]
+formats = []
+
+[[case]]
+name = "config-output-table-pins-format"
+args = ["stocks", "quote", "BBCA"]
+formats = []
+before = [["config", "set", "general.output", "table"]]
+
 [[case]]
 name = "cache-info-empty"
 args = ["cache", "info"]
@@ -78,10 +96,12 @@ args = ["stocks", "quote", "BBCA"]
 
 [[case]]
 name = "stocks-quote-multi"
+formats = ["table", "json", "jsonl"]
 args = ["stocks", "quote", "BBCA,BBRI"]
 
 [[case]]
 name = "stocks-history"
+formats = ["table", "json", "jsonl"]
 args = ["stocks", "history", "BBCA"]
 
 [[case]]
@@ -111,6 +131,7 @@ args = ["stocks", "risk", "BBCA"]
 
 [[case]]
 name = "stocks-fundamental"
+formats = ["table", "json", "jsonl"]
 args = ["stocks", "fundamental", "BBCA"]
 
 [[case]]
@@ -169,6 +190,38 @@ args = ["stocks", "screen", "--filter", "high-volume"]
 name = "stocks-screen-large-cap"
 args = ["stocks", "screen", "--filter", "large-cap"]
 
+[[case]]
+name = "auto-format-piped"
+args = ["stocks", "quote", "BBCA,BBRI"]
+formats = []
+
+[[case]]
+name = "auto-format-env-table"
+args = ["stocks", "quote", "BBCA"]
+formats = []
+env = { IDX_OUTPUT = "table" }
+
+[[case]]
+name = "stocks-quote-stdin"
+args = ["stocks", "quote", "-"]
+stdin = "BBCA\nBBRI\n"
+
+[[case]]
+name = "stocks-compare-stdin"
+args = ["stocks", "compare", "-"]
+stdin = "BBCA, BBRI"
+
+[[case]]
+name = "error-stdin-empty"
+args = ["stocks", "quote", "-"]
+stdin = ""
+
+[[case]]
+name = "error-invalid-output-env"
+args = ["stocks", "quote", "BBCA"]
+formats = []
+env = { IDX_OUTPUT = "yaml" }
+
 [[case]]
 name = "stocks-quote-offline-warm"
 args = ["--offline", "stocks", "quote", "BBCA"]
@@ -214,6 +267,7 @@ env = { IDX_MOCK_MSN_KEYRATIOS_FIXTURE = "tests/fixtures/msn_keyratios_industry_
 
 [[case]]
 name = "error-screen-region"
+formats = ["table", "json", "jsonl"]
 args = ["stocks", "screen", "--region", "eu"]
 
 [[case]]
@@ -298,6 +352,7 @@ ownership = true
 
 [[case]]
 name = "ownership-changes"
+formats = ["table", "json", "jsonl"]
 args = ["ownership", "changes", "--from", "2026-01-30", "--to", "2026-02-27"]
 ownership = true