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>

src/error.rs

@@ -76,8 +76,23 @@ impl IdxError {
         }
     }
 
+    /// Exit status by error class, so scripts can branch without parsing
+    /// stderr. Clap usage errors also exit 2. See docs/DESIGN.md.
     pub fn exit_code(&self) -> i32 {
-        1
+        match self.code() {
+            ErrorCode::InvalidInput | ErrorCode::ConfigError => 2,
+            ErrorCode::SymbolNotFound | ErrorCode::NoMarketData | ErrorCode::CacheMiss => 3,
+            ErrorCode::Http
+            | ErrorCode::RateLimited
+            | ErrorCode::ProviderUnavailable
+            | ErrorCode::Offline
+            | ErrorCode::AuthError => 4,
+            ErrorCode::Unsupported => 5,
+            ErrorCode::Io
+            | ErrorCode::ParseError
+            | ErrorCode::DatabaseError
+            | ErrorCode::PdfParseError => 1,
+        }
     }
 }