test: add golden output parity gate and live A/B compare

Phase 0 of docs/DESIGN.md, before any refactor code:

- tests/golden.rs runs every case in tests/golden/cases.toml (126
  outputs across help, completions, config, cache, all stocks commands
  on MSN and Yahoo mocks, error paths, and ownership queries on a seeded
  fixture DB) and compares exit code, stdout, and stderr byte for byte.
  Regenerate only with UPDATE_GOLDEN=1 for a reviewed output change.
- scripts/live-smoke.sh --compare <old-idx> runs each live case with
  both binaries and fails on differing exit codes, added/missing/retyped
  JSON keys, or changed values for slow-moving data.
- Exclude golden files from the whitespace/EOF pre-commit fixers, which
  would otherwise rewrite pinned help output.

Both gates were fault-injected: a one-word table change fails exactly
the three history goldens, and a binary dropping profile.long_name fails
only the profile live comparison.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

AGENTS.md

@@ -13,7 +13,7 @@ src/
 ├── ownership/  xlsx + pdf parsers, archive (ZIP/TXT), remote discovery,
 │               snapshot sync, db (SQLite/FTS5), entities, search, graph
 └── output/     table.rs, json.rs
-tests/cli.rs    CLI integration tests; fixtures in tests/fixtures/
+tests/          cli.rs (behavior), golden.rs + golden/ (output parity), fixtures/
 scripts/        smoke, installer, and ownership snapshot publisher
 ```
 
@@ -42,5 +42,5 @@ Docs:
 ```bash
 nix flake check -L                       # canonical gate (fmt, clippy, tests, package)
 nix develop --command cargo test <name>  # focused loop
-scripts/live-smoke.sh                    # opt-in live provider drift check
+scripts/live-smoke.sh --compare <old>    # live drift check, A/B against an older build
 ```