feat: add 5 engineering agents (Code Reviewer, Database Optimizer, Git Workflow, Software Architect, SRE)

[kienbui1995]

What

Adds 5 new agents to the Engineering Division, filling clear gaps in the current roster.

New Agents

Agent	Specialty	Why It's Needed
👁️ Code Reviewer	Constructive code review, security, maintainability	No existing agent covers code review
🗄️ Database Optimizer	Schema design, query optimization, indexing	Backend Architect is broad; this is DB-specialized
🌿 Git Workflow Master	Branching strategies, conventional commits, advanced Git	No Git-focused agent exists
🏛️ Software Architect	System design, DDD, architectural patterns, ADRs	No high-level software design agent
🛡️ SRE	SLOs, error budgets, observability, chaos engineering	Incident Response is reactive; SRE is proactive reliability

Cross-Division Overlap Check

• ✅ No overlap with Testing division (QA/verification vs dev practices)
• ✅ No overlap with Support division (Infrastructure Maintainer is ops, not SRE)
• ✅ No overlap with Project Management (Jira Workflow Steward is Jira-linked, not Git)
• ✅ Database Optimizer complements Backend Architect (depth vs breadth)

Context

Split from #124 per maintainer feedback. Focused PR with only engineering agents.

Testing

• All 5 files follow existing frontmatter format (name, description, color, emoji, vibe)
• README updated with new roster entries