Document/guard no-newline limitations for text/code output

[kantord]

Summary

--no-newline currently only has an effect for structured templates (JSON/YAML). In text/code modes the renderer always emits line-oriented output with a trailing newline, so the flag silently does nothing. The CLI help/README don’t mention this limitation, which can confuse users.

Steps to Reproduce

1. Run headson --no-newline -f text tests/fixtures/code/sample.py.
2. Observe that the output still ends with a newline and spans multiple lines.

Expected Behavior

Documentation (and ideally the CLI) should state that --no-newline only applies to structured formats. When the flag is used with text/code output, the tool should warn the user or explicitly ignore it.

Proposed Fix

• Update the README and --help text to note that --no-newline is only honored for JSON/YAML-family templates.
• Consider emitting a warning when --no-newline is combined with -f text, -f code, or --format auto that resolves to those modes.