[SPARK-56378] Add PR description guidelines to AGENTS.md

[szehon-ho]

What changes were proposed in this pull request?

Require PR descriptions to use the section headings from .github/PULL_REQUEST_TEMPLATE in AGENTS.md (AI agent instructions).

Why are the changes needed?

The previous instruction ("Follow the template") was too vague and resulted in AI-generated PRs that did not follow the template format.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

No tests needed — documentation-only change.

Was this patch authored or co-authored using generative AI tooling?

Generated-by: Claude (Cursor agent mode)

[szehon-ho]

fyi @peter-toth @cloud-fan , i tried AGENTS.md to make the pr #55179

but it looked weird initially:

Summary

1. Converts partition-column filters to PartitionPredicates (reusing SPARK-55596 infrastructure)
2. Translates remaining data-column filters to standard V2 predicates
3. Combines them (partition predicates first) and calls table.canDeleteWhere

Changes

• OptimizeMetadataOnlyDeleteFromTable: Added tryDeleteWithPartitionPredicates fallback method and tryTranslateToV2 helper
• PushDownUtils: Extracted createPartitionPredicates and made flattenNestedPartitionFilters package-private for reuse; getPartitionPredicateSchema now returns None for empty partition fields
• InMemoryTableWithV2Filter: Extracted evalPredicate to companion object for reuse by test tables
• InMemoryPartitionPredicateDeleteTable (new): Test table supporting PartitionPredicates and configurable data predicate acceptance
• DataSourceV2EnhancedDeleteFilterSuite (new): 9 test cases covering first-pass accept, second-pass accept/reject, mixed partition+data filters, UDF on non-contiguous partition columns, multiple PartitionPredicates, and row-level fallback

Test plan

• DataSourceV2EnhancedDeleteFilterSuite — 9/9 pass
• DataSourceV2EnhancedPartitionFilterSuite — 19/19 pass (no regressions)
• GroupBasedDeleteFromTableSuite — 32/32 pass (no regressions)
• Scalastyle — 0 errors

So want to clarify to make AGENTS use the right template

[szehon-ho]

actually im not 100% sure , it already mentioned the template, not sure why it didnt follow

[cloud-fan]

actually I already have a fix here: #55139