feat: v2.4.0 — protocol enhancements & coordination patterns

[ducdmdev]

Summary

• 2 new protocol message types: PROGRESS and CHECKPOINT for long-running task visibility
• 2 protocol enhancements: confidence grades [X%] on findings, priority marking on STARTING/HANDOFF
• 5 new coordination patterns: Checkpoint/Rollback, Deadline Escalation, Circular Dependency Detection, Graceful Degradation, Warm vs Cold Handoff
• Anti-Pattern Catalog: 8 documented coordination pitfalls (4 critical, 4 warning) with cross-references
• Scaling Patterns: Read-only extension (5-6 agents), phased execution (12-16+), sub-agent specialization
• Cross-cutting updates: lead processing rules, README, CLAUDE.md research reference
• Version bump: 2.3.0 → 2.4.0 with full CHANGELOG

All changes are documentation-only. No hook scripts, SKILL.md frontmatter, or code changes.

Based on research from team 0309-protocol-research (4 parallel research streams covering multi-agent frameworks, distributed systems, organizational theory, and coordination patterns).

Test plan

• Full test suite passes (9 files, 102 assertions)
• claude plugin validate . passes
• New PROGRESS/CHECKPOINT message types present in shared-phases.md lead processing rules
• All 5 new coordination patterns present in coordination-patterns.md with Contents links