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>

README.md

@@ -112,7 +112,11 @@ idx ownership changes --from 2026-07-31 --to 2026-08-31
 
 ### Output
 
-- Human-readable tables on stdout, JSON with `-o json`, errors on stderr
+- Tables when you run idx in a terminal; compact JSON when its output is piped
+  or captured, so scripts and AI agents get structured data with no flag
+- `-o table|json|jsonl` (or `IDX_OUTPUT`, or `general.output` in the config)
+  overrides that; `jsonl` prints one array item per line
+- Errors go to stderr, as one JSON object in JSON modes
 
 ## Configuration
 
@@ -133,12 +137,26 @@ Precedence order:
 3. Config file
 4. Built-in defaults
 
-## Agent-friendly usage
+## Scripting and agents
 
-- Use `idx --help` and subcommand help for discoverability
-- Use `-o json` / `--output json` for structured output
+idx composes with `jq`, Python, and other shell tools:
+
+```bash
+idx stocks quote BBCA BBRI | jq -r '.[] | "\(.symbol) \(.price)"'
+idx -o jsonl stocks history BBCA --period 1y | tail -5
+idx stocks screen --filter low-pe | jq -r '.[].symbol' | idx stocks compare -
+```
+
+- stdout carries only data; warnings and errors go to stderr
+- `-` reads symbols from stdin for `stocks quote` and `stocks compare`
+- No color codes when output is not a terminal
+- Exit codes: `0` success, `2` invalid input or config, `3` not found or no
+  cached data, `4` upstream unavailable (retry later), `5` unsupported, `1` other
 - `idx completions <shell>` generates shell completions
 
+If you ran `idx config init` before v0.3, your config pins `output = "table"`;
+change it to `auto` with `idx config set general.output auto`.
+
 ## Documentation
 
 - [docs/INSTALL.md](docs/INSTALL.md): install script options