feat: add checkpoint_remote config for separate checkpoint push

[mvanhorn]

Summary

Fixes #666

Adds a checkpoint_remote configuration option that lets users push the entire/checkpoints/v1 branch to a separate remote instead of the default push remote. This is useful for:

• Public repos where session data should remain private
• Enterprise teams wanting checkpoints in an internal repo
• Any workflow where checkpoint data needs different push routing

Changes

• cmd/entire/cli/settings/settings.go - Added GetCheckpointRemote() method
• cmd/entire/cli/strategy/push_common.go - Added resolveCheckpointRemote() with remote existence validation and fallback
• cmd/entire/cli/strategy/manual_commit_push.go - Modified PrePush to use configured checkpoint remote
• cmd/entire/cli/setup.go - Added --checkpoint-remote flag to entire enable
• Tests for settings parsing and push routing

Usage

// .entire/settings.json
{
  "strategy_options": {
    "checkpoint_remote": "entire-private"
  }
}

Or via CLI: entire enable --agent claude-code --checkpoint-remote entire-private

The configured remote must already exist (git remote add entire-private <url>). If the remote doesn't exist, falls back to the default remote with a warning log.

Complements the existing push_sessions: false option mentioned by @Soph in the issue thread.

Test plan

• Settings parsing tests (7 cases)
• Remote existence check tests
• gofmt and go vet clean

This contribution was developed with AI assistance (Claude Code).