docs: add user guide and example projects (#139)

New users were confused about which files they write vs. which Ralph
manages, and how PROMPT.md, specs/, and fix_plan.md relate to each other.

Added:
- docs/user-guide/ with quick start tutorial, file reference, and
  requirements writing guide
- examples/simple-cli-tool/ showing minimal Ralph configuration
- examples/rest-api/ demonstrating when to use specs/
- README section explaining Ralph files and their relationships

Also documents specs/stdlib/ purpose for reusable patterns.

examples/simple-cli-tool/.ralph/PROMPT.md

@@ -0,0 +1,57 @@
+# Ralph Development Instructions
+
+## Context
+You are Ralph, building a command-line todo application in Node.js. This is a personal productivity tool that stores tasks locally and provides simple commands for task management.
+
+## Current Objectives
+1. Create a CLI that supports add, list, complete, and delete commands
+2. Store todos in ~/.todos.json with automatic file creation
+3. Provide clear, helpful output for all operations
+4. Handle errors gracefully with actionable messages
+
+## Technology Stack
+- Node.js 18+
+- commander.js for CLI argument parsing
+- Native fs/promises for file operations
+- Jest for testing
+
+## Key Principles
+- Single responsibility: each command does one thing well
+- Fail gracefully: missing file = empty list, not an error
+- Clear output: users should always know what happened
+- Testable: core logic separated from CLI layer
+
+## Command Specifications
+
+### `todo add "task description"`
+- Adds a new task with auto-incrementing ID
+- Outputs: "Added task #3: Buy groceries"
+
+### `todo list`
+- Shows all tasks with status indicators
+- [ ] for pending, [x] for completed
+- Outputs: "No tasks yet" if empty
+
+### `todo complete <id>`
+- Marks task as done
+- Errors if ID doesn't exist
+
+### `todo delete <id>`
+- Removes task permanently
+- Errors if ID doesn't exist
+
+## Data Format
+```json
+{
+  "nextId": 4,
+  "tasks": [
+    {"id": 1, "text": "Buy groceries", "completed": false},
+    {"id": 2, "text": "Call mom", "completed": true}
+  ]
+}
+```
+
+## Quality Standards
+- All commands have --help documentation
+- Unit tests for storage module
+- Integration tests for CLI commands