feat: add token-based output splitting

[dyoshikawa-claw]

Summary

• add --max-tokens to split rendered JSON output by token count
• keep output chunking in date order while honoring the stricter of size/token limits
• document the new behavior and add tests for token-based chunking

Details

• token counting uses github.com/tiktoken-go/tokenizer with o200k_base
• if a single rendered event exceeds --max-tokens, the command returns an error

Validation

• go mod tidy
• go test ./...
• go build ./cmd/ghactivities