Docs: Add new Spark version support skill

[manuzhang]

About this PR

Adds a repo-local, agent-neutral skill for introducing support for a new Spark minor version in Iceberg.

The skill generalizes the Spark 4.2 preview workflow and documents how to:

• preserve source history with separate move and copy-back commits;
• back up an existing target branch before rewriting its history;
• update the current Gradle, CI, benchmark, and publication integration points;
• keep preview source support separate from default build and release publication decisions; and
• validate the new modules and an older Spark line when shared resolution logic changes.

Testing

• quick_validate.py .agents/skills/add-spark-version
• git diff --check upstream/main...HEAD

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AI Disclosure

• Model: GPT-5
• Platform/Tool: OpenAI Codex
• Human Oversight: partially reviewed
• Prompt Summary: Create a reusable, agent-neutral Iceberg skill based on the Spark 4.2 preview workflow for adding support for new Spark versions, including backup of an existing target branch.