Merge in claude-code skill and bump version (0.3.9)

[aliroberts]

Summary

• Add --output plain mode for machine-readable output suitable for LLM agents
• Add experimental weco setup claude-code command to install Weco skill for Claude Code
• Update README with missing CLI commands (login, credits, setup)

Plain Output Mode

New --output flag for weco run and weco resume commands:

weco run --output plain --source optimize.py ...

Mode	Description
rich (default)	Interactive terminal UI with colors, progress bars, and sparklines
plain	Structured text output without ANSI codes, designed for LLM agent consumption

The plain mode:

• Uses bracketed tags like [STEP 1/10], [METRIC], [EXECUTION OUTPUT START]
• Outputs full execution results (not truncated) for agent analysis
• Flushes output immediately for real-time streaming

Setup Command (Experimental)

New command to install Weco skills for AI coding assistants:

weco setup claude-code

This:

1. Clones the weco-skill repo to ~/.claude/skills/weco/
2. Prompts to update ~/.claude/CLAUDE.md with skill discovery instructions
3. Enables Claude Code to automatically suggest Weco for optimization task