Add a 'becwright list' command to discover built-in checks

[DataDave-Dev]

Context

There is no way to discover the available built-in checks (or the catalog BECs)
from the CLI — you have to read the docs. A becwright list command would make
the tool more discoverable and lower the barrier to writing the first rule.

Proposed approach

• Add a list subcommand in src/becwright/cli.py, following the existing
  sub.add_parser(...).set_defaults(func=...) pattern.
• Print the built-in checks under src/becwright/checks/, one per line, each with
  a short description.
• Optionally also list the catalog BECs in becs/ (or print the catalog URL).

Acceptance criteria

• becwright list prints the built-in checks.
• Covered by a test in tests/ (e.g. assert a known check name appears).

Notes

Good first issue — small and self-contained.

Relevant files

src/becwright/cli.py, src/becwright/checks/.