Added Lambda-runtime-upgrade-skill

[higoyal01]

What

Adds a portable /lambda-runtime-upgrade skill that packages the Lambda
runtime upgrade workflow so it can be used independently in any
MCP-compatible or shell-capable agent (Claude Code, Kiro, DevOps agents),
not just within this system.

Why

The lambda-runtime agent + MCP tools already exist (PR #9). This wraps that
same capability as a portable skill, matching the pattern of the existing
skills (finops-analysis, health-events-digest, tag-governance-assessment,
resilience-report).

Details

• skills/lambda-runtime-upgrade/SKILL.md — three-path routing
  (MCP tools → AWS CLI → delegate), matching the existing skills
• Upfront region-selection Q&A to scope work before scanning (speed)
• Parallel multi-region scanning: Path M via
  get_deprecated_functions_multi_region, Path A via background shell jobs
• reference/runtime-migration-kb.md — self-contained deprecation dates +
  per-family breaking changes (Python, Node.js, Java, .NET, Ruby, custom)
• Read-only guardrails; recommends upgrades, never mutates functions
• Reuses the existing lambda-runtime MCP tools — no new backend code
• Registered in the README skills table

Testing

• Skill frontmatter keys match the other skills
  (name, description, argument-hint, allowed-tools, user-invocable)
• No new Python/backend code, so no unit-test impact