feat(plugins): add prompt_newline plugin with /prompt_newline toggle

[mmcguff]

🐶 feat(plugins): add prompt_newline plugin with /prompt_newline toggle

What

Adds a tiny opt-in ergonomics plugin that drops user input onto a fresh line below the puppy/agent/model/cwd prompt chrome. Helpful when your cwd is a 90-character monstrosity and there's no horizontal room left to type.

Before (default, unchanged):

🐶 puppy [code-puppy 🐶] [gpt-5] (~/long/path/to/project) >>> my task here

After enabling /prompt_newline on:

🐶 puppy [code-puppy 🐶] [gpt-5] (~/long/path/to/project) >>>
my task here

Why

Long working-directory paths squeeze the input area to nothing on standard terminal widths. A trailing newline on the prompt is the cheapest possible fix without redesigning the chrome. Some folks love it, some hate it — so it's off by default and toggleable per user, persisted in puppy.cfg.

How (per CONTRIBUTING.md)

This is a plugin, not a core change. Lives entirely under code_puppy/plugins/prompt_newline/ and uses three documented hooks from code_puppy/callbacks.py:

Hook	Purpose
startup	Monkey-patches get_prompt_with_active_model to optionally append ("", "\n") to the returned FormattedText. The wrapper re-checks is_enabled() at call time, so the slash-command toggle takes effect immediately — no restart.
custom_command	Handles /prompt_newline [on|off|toggle]. Bare /prompt_newline flips the current state.
custom_command_help	Adds the command to /help.

State is persisted to puppy.cfg via the existing get_value / set_config_value helpers (mirrors the dbos_durable_exec plugin pattern). Default is OFF — opting in is a one-line PR-from-the-user's perspective.

The monkey-patch is idempotent (guarded by a sentinel attribute) and fail-soft (any exception in the wrapper falls back to the original FormattedText, never crashes the prompt).

Files

code_puppy/plugins/prompt_newline/__init__.py            # docstring only
code_puppy/plugins/prompt_newline/config.py              # is_enabled / set_enabled
code_puppy/plugins/prompt_newline/register_callbacks.py  # hook registration
tests/plugins/test_prompt_newline_plugin.py              # 12 test cases

Total: 378 LOC (well under the 600-line cap; no file approaches it).

Tests

12 new test cases in tests/plugins/test_prompt_newline_plugin.py covering:

• /help entry registration
• Custom-command name routing (ignores other commands)
• All toggle-arg parsing variants (on/off/true/false/1/0/yes/no/enable/disable/toggle)
• Garbage-arg rejection
• _append_newline purity (doesn't mutate the original FormattedText)
• _install_prompt_patch idempotency + original-function preservation
• Patched prompt appends newline only when is_enabled() returns True
• Explicit on persists to puppy.cfg
• Bare /prompt_newline flips state correctly
• Default state is OFF
• Garbage arg surfaces an error and doesn't change config

50 passed, 1 xpassed in 11.94s

(Also re-ran tests/test_prompt_toolkit_completion.py to confirm zero regressions in the patched module.)

Lint

ruff check --fix and ruff format both clean.

Backwards compatibility

✅ Zero behavior change for existing users. Default is OFF; users must opt in with /prompt_newline on.

Future work (not in this PR)

If demand materializes, the same plugin could grow:

• Env-var override (CODE_PUPPY_PROMPT_NEWLINE=1)
• /set prompt_newline true integration via the existing settings completer
• Configurable separator (e.g. blank line vs ↵ glyph)

Intentionally YAGNI'd for now.