Environment variable manager for MontRS. Parses the [env] section from montrs.toml, renders Tera templates, loads .env files, and applies environment variables to the process.

• Parse [env] section: FOO = "bar", FOO = { value, export, redact, required }, _.path
• Tera template rendering in env values with env/cwd custom functions
• .env file loading with layered resolution
• EnvDiff computation for set/unset detection
• montrs env list, montrs env set, montrs env unset, montrs env exec CLI commands
• PATH manipulation via _.path directive (prepend, append, remove)

[env]
RUST_BACKTRACE = "1"
MONTRS_LOG = "info"
MY_VAR = { value = "secret", export = false }
_.path = ["/path/to/bin"]