feat: Add CLI argument parsing for agent selection and initial messages

[CSRessel]

Summary

This PR adds command-line argument parsing to nori-cli, allowing users to specify the agent and initial message from the command line.

Changes

• Add clap dependency: Added clap v4 with derive feature for CLI argument parsing
• New CLI module: Created src/cli.rs with Cli struct and agent name mapping functions
• Main function updates: Modified main.rs to parse CLI args, validate agent names, and read from stdin when piped
• Run app updates: Updated run_app to accept optional agent_index and initial_message parameters
• Comprehensive tests: Added tests/cli_args_test.rs with tests for all CLI parsing scenarios
• Documentation updates: Updated docs in both src/docs.md and tests/docs.md

Usage Examples

# Use specific agent
nori-cli --agent claude

# Use specific agent with initial message
nori-cli --agent codex "Hello world"

# Pipe input as initial message
echo "Hello from stdin" | nori-cli

# Short flag for agent
nori-cli -a mock "Test message"

Features

• Agent selection via --agent or -a flag (claude, codex, claudecode, mock)
• Initial message as positional argument or via stdin piping
• Case-insensitive agent name matching
• Input validation with helpful error messages
• CLI arguments take precedence over stdin when both provided