feat: add token counting (OpenAI tiktoken + Claude API)

[kiki830621]

Summary

Add macdoc convert --to tokens <file> that outputs exact token counts.

Depends on #27 (CLI migration to convert --to pattern).

CLI Interface

macdoc convert --to tokens file.md                        # both models
macdoc convert --to tokens --model gpt-4o file.md         # OpenAI only
macdoc convert --to tokens --model claude-sonnet file.md   # Claude only

Output Format

# Default (tabular)
Model              Tokens
─────────────────────────
gpt-4o             1,234
claude-sonnet-4-6  1,198

# Single model (just the number for piping)
1234

Token Counting Methods

Provider	Method	Cost
OpenAI	swift-tiktoken (offline BPE)	Free, offline
Claude	Anthropic /v1/messages/count_tokens API	Free, online (RPM-limited)

Architecture

• New package: token-counter-swift under packages/
• Layer 2 utility (measurement tool, not format converter)
• Depends on: swift-tiktoken, Foundation (URLSession for Anthropic API)
• Needs ANTHROPIC_API_KEY env var for Claude counting

References

• Anthropic token counting docs — free, RPM-limited
• Blocked by refactor: unify CLI to macdoc convert --to pattern (textutil-compatible) #27