Tracer: `fafycat cat list` end-to-end

[davidchris]

Parent

#26

What to build

The first vertical slice of the read-only query CLI. Adds a single subcommand fafycat cat list that returns all categories as JSON. Establishes the patterns every later slice will reuse:

• Nested argparse subparsers (cat namespace).
• JSON-only stdout output with indent=2.
• Error contract: {"error": "..."} to stdout with non-zero exit; argparse usage to stderr with exit 2.
• Logging suppressed below ERROR for query handlers.
• Subprocess-based integration tests against a temporary database.
• Reuse of CategoryService.get_categories from the existing service kernel — no new business logic.
• Reuse of --data-dir and FAFYCAT_DATA_DIR resolution via the existing _apply_data_dir_override helper.

A small shared output helper module is introduced here so later slices share the same JSON / exit-code framing.

Acceptance criteria

• Running fafycat cat list returns a JSON array of category objects on stdout, exit 0.
• Running fafycat cat list --include-inactive includes inactive categories in the result.
• Running fafycat cat list --help exits 0 and prints usage including at least one example invocation.
• Running fafycat cat --help lists the list subcommand.
• Stdout is valid JSON parseable by json.loads.
• Stdout is indent=2 formatted.
• Stderr is empty on the happy path.
• Argparse errors (e.g. unknown flag) exit 2 with usage on stderr.
• Runtime errors (e.g. missing DB) emit {"error": "..."} to stdout and exit 1.
• Existing fafycat serve, fafycat import, fafycat init commands continue to work unchanged.
• Tests are written before the implementation (red-green TDD).
• Subprocess integration tests cover happy path, --include-inactive, missing-DB error path.
• No assertions on log output, internal call order, or private attributes.

Blocked by

None - can start immediately.