Add release docs and skills

[zgeorgiev-smartsheet]

Pull Request

Summary

Add release documentation and AI agent workflow skills for the Smartsheet Java SDK. This introduces:

• RELEASE.md — Step-by-step release procedure covering version bumps, changelog updates, tagging, and GitHub Release publishing.
• AGENTS.md — Defines available AI agent workflows (Implementation, Review, Release) with their purposes, skill references, and shared project context.
• CLAUDE.md — Root-level Claude Code configuration pointing to AGENTS.md.
• Release skill (.claude/skills/releasing-smartsheet-java-sdk/SKILL.md) — Detailed skill file backing the Release Agent workflow.

Checklist

• Read the contribution guide
• Successfully build your changes locally - Run ./gradlew clean build
• Include a title that clearly describes your changes and references relevant issues / backlog items
• Include a summary of your changes
• Include tests for your changes where possible — N/A, documentation-only changes