feat: model escalation for task-group-implementer (sonnet→opus)

[mariuszs]

Summary

• Enriched task-group-implementer agent with BLOCKED/NEEDS_CONTEXT status protocol and model: sonnet default
• Added automatic model escalation (sonnet → opus) in implementation-plan-executor when subagent reports BLOCKED
• Documented the model escalation pattern in orchestrator-patterns.md for future reuse

Changes

File	Change
agents/task-group-implementer.md	5-status protocol, model: sonnet, escalation guidance
skills/implementation-plan-executor/SKILL.md	Status handling, opus escalation, re-dispatch template
orchestrator-framework/references/orchestrator-patterns.md	New section 7: Model Escalation Pattern
CLAUDE.md	Agent/skill table entries, principle #9

Test plan

• Run /maister:development with a complex task to verify task-group-implementer dispatches on sonnet
• Simulate BLOCKED response to verify coordinator escalates to opus
• Verify make validate passes