Config by Prompt

# Config by Prompt

Status: Implemented

## Overview

### Purpose

- Define how a prompt markdown file can override agent runtime settings using YAML front matter.
- Define how a config file can set per-prompt `model` and `agent-mode` overrides in a dedicated section.
- Ensure front matter metadata is stripped before prompt text is sent to the external agent CLI.

### Goals

- Support per-prompt overrides for `model` and `agent-mode`.
- Support per-prompt overrides from both front matter and config file.
- Keep existing global configuration behavior intact.
- Define deterministic precedence across flags, env vars, front matter, config file, and defaults.
- Specify clear behavior for invalid front matter and unknown keys.

### Non-Goals

- Introducing new prompt source types.
- Adding support for overriding `agent` in front matter.
- Defining implementation-specific parser internals.

### Scope

- In scope: markdown prompt files loaded via `--prompt-file` and `PromptsDir/<prompt>.md`.
- Out of scope: inline prompts (`--prompt`) and stdin prompt text (`-`).

## Architecture

### Module/package layout (tree format)