feat: Add CLI backend for Claude Max subscription

Add support for using Claude Code CLI as an alternative backend,
allowing users with Claude Max subscriptions to use llm9p without
API tokens.

New files:
- internal/llm/backend.go: Backend interface for swappable LLM providers
- internal/llm/cli_client.go: CLI-based client using `claude` command

Changes:
- Add -backend flag: 'api' (default) or 'cli'
- Refactor llmfs to use Backend interface instead of concrete Client
- Model names normalized for CLI (opus, sonnet, haiku)

Usage:
  ./llm9p -backend cli  # Uses Claude Max subscription
  ./llm9p -backend api  # Uses Anthropic API (default)

Limitations of CLI backend:
- Token counting not available (always 0)
- Streaming is simulated (full response as single chunk)
- Uses short model names (opus, sonnet, haiku)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>