sessionSecret was previously baked directly into the scaffolded config file;
`triggershell init` now generates a .env with TRIGGERSHELL_SESSION_SECRET
instead and references it via ${VAR} interpolation, keeping the actual
secret out of the (often committed) config file. `triggershell dev/start/
validate` load that .env automatically without overriding real env vars.
Also renames the default config filename from triggershell.config.yaml to
triggershell.yml throughout the CLI, app, docs, and examples.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
