docs: document Spark version labels in bug triage guide

[andygrove]

Which issue does this PR close?

Closes #.

Rationale for this change

Comet has per-version spark 4.0, spark 4.1, and spark 4.2 labels in the issue tracker, but the bug triage guide does not tell triagers when to apply them. Without a documented convention, version-specific bugs get filed without a version label and are harder to surface in version-scoped queries (for example, when prepping a Spark 4.1 release or auditing 4.1-only failures).

What changes are included in this PR?

• Add a "Spark Version Labels" section to docs/source/contributor-guide/bug_triage.md describing when to apply spark 4.0, spark 4.1, or spark 4.2.
• Add a step to the "For New Issues" checklist for applying a Spark version label.
• Drop the stale spark 4 entry from the area-indicators line since the actual labels are version-specific.
• Update the bug-triage Claude skill (.claude/skills/bug-triage/SKILL.md) to mirror the same guidance: a new classification step, an updated gh issue edit example, and an optional Spark version labels: line in the per-issue summary template.

This work was scaffolded with the bug-triage project skill.

How are these changes tested?

Documentation-only change. No code is affected.