pub fn validate_alias_key(key: &str) -> Result<(), String>Expand description
Validate that an alias key is safe for use in TOML dotted paths, URLs,
filesystem paths on Windows/macOS/Linux, and ZEROCLAW_* env-var grammar.
Allowed: lowercase ASCII alphanumeric plus single underscore, 1-63 chars.
Must start AND end with alphanumeric. Adjacent underscores (__) are
forbidden because they collide with the env-var grammar’s path separator.
The env-var grammar uses __ as path separator, which lets aliases keep
single _ literally (prod_v2, staging_api). Hyphens are forbidden
because they are illegal in POSIX env-var identifiers; uppercase is
forbidden so the bootstrap env-vars (ZEROCLAW_WORKSPACE,
ZEROCLAW_CONFIG_DIR) stay disambiguated by case.