feat: add AGENTS.md and CLI agent skills

[joshrotenberg]

Summary

• AGENTS.md in repo root: gives coding agents context for working on the redisctl codebase (build/test commands, crate architecture, conventions, macro patterns, CI requirements)
• 7 CLI agent skills in skills/ following the agentskills.io spec: teach agents how to use the redisctl CLI for Redis management tasks

Skills

Skill	Description
redisctl-setup	Installation, profile creation, shell completions
redisctl-cloud-management	Cloud subscription/database lifecycle, tasks, cost reports
redisctl-enterprise-ops	Cluster status, database management, stats, logs, CRDBs
redisctl-enterprise-admin	RBAC, LDAP, licensing, certs, proxy, diagnostics
redisctl-networking	VPC peering, Transit Gateway, PSC, PrivateLink
redisctl-database-connect	redis-cli sessions, multi-cluster profiles, raw API
redisctl-workflows	End-to-end provisioning, backup/recovery, migration patterns

Context

• Complements existing MCP skills in crates/redisctl-mcp/skills/ (index-advisor, query-tuning, index-ab-test)
• AGENTS.md helps agents work on redisctl; skills help agents use redisctl
• Skills are designed for eventual inclusion in redis/agent-skills
• See also: agents.md standard for project-level agent context

Ref #883

Test plan

• Verify AGENTS.md renders correctly on GitHub
• Verify all SKILL.md files have valid frontmatter (name + description)
• Skill directory names match frontmatter name field