docs: add upgrade-datafusion skill [WIP]

[andygrove]

Summary

Adds a Claude Code skill (upgrade-datafusion) that walks a maintainer through creating or refreshing a WIP branch tracking apache/datafusion main. The skill's purpose is to make forward-looking DF upgrade PRs (like #4888) a repeatable, low-friction operation so CI runs continuously against upstream DataFusion and breaking changes surface early.

Key content in the skill:

• Detects an existing WIP upgrade PR so refreshes don't spawn duplicates.
• Pins Comet's arrow/parquet/object_store to whatever DataFusion's root Cargo.toml uses at the target rev.
• Points the maintainer at DataFusion's per-release upgrade guides (docs/source/library-user-guide/upgrading/*.md) so breaking-API fixes are grounded in upstream documentation rather than guesswork.
• Lists common upstream breakages Comet has hit before (GroupsAccumulator::merge_batch, MutableArrayData::extend deprecation, etc.).
• Includes a PR body template so every WIP upgrade PR looks consistent.

No runtime code changes. Skill is opt-in — the user has to invoke it (or ask for a "DataFusion upgrade PR").

Test plan

• Skill file is discovered by Claude Code (appears in the available-skills list)
• Follow the skill on a synthetic upgrade to confirm each step is actionable