fix: require explicit billing approval marker

[Naniko0213]

Summary

• declare required environment and binaries using ClawHub/OpenClaw metadata
• require --user-approved before billing confirmation can make any network request
• emit billing approvals as user-visible events with an audit message
• update workflow and troubleshooting documentation
• bump the GitHub package version to 0.3.15

Verification

• Agent Skill validation passes with machine-readable requirements
• all shell scripts pass syntax and help checks
• billing confirmation without the approval marker exits before network access
• production endpoint remains pinned to https://pexo.ai
• ClawHub channel package differs only in channel identity and version