Add detect-secrets scanning

[reevejd]

Sets up IBM detect-secrets with an audited baseline and a pre-commit hook.

https://jsw.ibm.com/browse/SKILLS-131558

Findings

.github/workflows/rspec-tests.yml:73: Basic Auth Credentials, not a secret, local-only connection string (root:password@127.0.0.1)
docker-compose.yml:6: Secret Keyword, not a secret, obvious placeholder ("password" literal)
spec/dummy/config/database.yml:23: Basic Auth Credentials, not a secret, local-only connection string (postgres:password@localhost)
spec/dummy/config/database.yml:68: Basic Auth Credentials, not a secret, local-only connection string in comment (myuser:mypass@localhost)
spec/spec_helper.rb:13: Basic Auth Credentials, not a secret, local-only connection string (postgres:password@localhost)

Repo state

• Path taken: P (pre-commit framework)
• Package manager: none (Ruby/Gem project)
• Husky version: none
• core.hooksPath (local): (none)
• Baseline: 5 findings, 5 audited, 0 real
• Hook test: secret blocked (exit 1), clean file allowed (exit 0)