fix: harden pexo-agent release safety

[Naniko0213]

Summary

• remove API-key placeholder literals that ClawHub flags as exposed secrets
• disclose shell, network, file-transfer, and external-data boundaries
• require confirmation for every billable batch by default and remove the no-confirmation mode
• bump the GitHub package version to 0.3.13

Verification

• Agent Skill validation passes
• all shell scripts pass syntax and help checks
• billing mode tests confirm always is the default, threshold is opt-in, and never is rejected
• ClawHub channel package contains no API-key literal examples