Closes#258.
`egress_render_routes` and `_render_match_entry` now pass all manifest
strings (host, auth_scheme, token_env, path/header values) through
`_yaml_str_escape` before interpolating into double-quoted YAML scalars,
preventing stray `"` or newlines from corrupting routes.yaml.
`git_gate_render_gitconfig` now calls `_gitconfig_validate_value` on
each Upstream value (and the derived alias) before writing the
`insteadOf` line, rejecting any value containing a newline that would
inject arbitrary gitconfig keys.
didericis-claude