Catalog: Shell BECs (no-set-x-left-in)

[DataDave-Dev]

Part of #25 (one language per PR). No code required — a catalog BEC is a single YAML file using the generic forbid check.

Task

Add a ready-to-import Shell BEC under src/becwright/becs/:

• no-set-x-left-in.bec.yaml — forbid set -x tracing left enabled in **/*.sh (it leaks env values — including secrets — into logs).

How

1. Copy the format from src/becwright/becs/no-debug-go.bec.yaml.
2. Write a real intent and why_it_matters (the log-leak angle is the why).
3. Use check: "becwright run forbid --pattern '...'" with paths **/*.sh.
4. Add the entry to src/becwright/becs/README.md.

Acceptance criteria

• pytest green (test_catalog_bundles_are_valid covers bundle parsing).
• becwright add no-set-x-left-in installs it in a scratch repo.

See CONTRIBUTING.md → "Adding a catalog BEC".

[PedroVBAmorim]

Hello, I would like to work on this

[DataDave-Dev]

@PedroVBAmorim Excellent, you can't work in it