feat: add rebel skill - project introspection & goal guardian

[MycatQieZi]

Summary

Add the Rebel (叛逆者) skill — a critical observer that audits AI-assisted projects from outside the main coding loop.

• 4-Dimension Assessment: Architecture Health (30%), Code Quality (25%), Process Compliance (20%), Non-Functional Requirements (25%) — each scored 0-100
• 5 Crisis Levels (L0-L4): Peace → Watch → Warning → Crisis → Emergency, with auto escalation/de-escalation rules
• 3 Audit Modes: Micro (silent 2s check), Standard (full review every 5 tasks), Deep (comprehensive scan every 20 tasks)
• Coup Proposal System: Structured YAML proposals with diagnosis, evidence, and remediation — all requiring user approval
• Adaptive Sampling: Frequency adjusts based on audit results
• YAML-Driven Config: Fully customizable via .rebel/config.yaml

Skill source: https://github.com/MycatQieZi/rebel

Test Plan

• Verify SKILL.md frontmatter follows required schema
• Verify category and tags are appropriate
• Test install via the provided installCommand
• Confirm skill triggers correctly with /rebel commands