feat(harness): integrate harness plugin into monorepo

[moukrea]

Summary

• Moves the harness agent orchestration system from a standalone harness/ directory into plugins/harness/, following the established plugin conventions
• Adds plugin.json with hooks and skills references, registers in marketplace.json at v1.0.0
• Creates CLAUDE.md with 10 behavioral rules (complexity classification, test protection, failure gates, agent coordination, etc.)

What's included

• 9 agent definitions: architect, debugger, implementer, integrator, monitor, researcher, reviewer, tester, ui-verifier
• 12 skills: harness-init, session-bridge, task-analyze, task-decompose, requirements-interview, spec-ingest, verify-work, progress-report, recovery, harness-reflect, deployment-monitor, logs
• 16 hook scripts: session start, prompt classification, write guards, post-edit verification, failure tracking, stop gates, compaction handling, and more
• 8 rule templates: database, frontend, go, infrastructure, python, rust, testing, typescript