[MINOR][DOC] Suggest where to place new class members in AGENTS.md

[szehon-ho]

What changes were proposed in this pull request?

Adds one paragraph to the ## Development Notes section of AGENTS.md suggesting where to place a new member when editing an existing class: a private helper is better placed after the public methods that use it, or with the file's existing helpers, rather than inserted between two public methods or between two test(...) blocks.

It is phrased as a suggestion with a default, not a rule. Files in the repo vary in how they order members, so the note explicitly defers to the local convention of whatever file is being edited and does not ask anyone to reorganize existing code.

Why are the changes needed?

Agents writing code top-down tend to define a helper immediately above the first method that uses it, which lands it in the middle of a class's public surface or a suite's list of test cases and interrupts a top-to-bottom read. I hit exactly this while writing a test suite and had to move the helper in review.

This fits the existing pattern of the surrounding notes, which call out specific, recurring failure modes (non-ASCII characters in comments, overrunning the 100-character line limit) rather than restating general style guidance. Unlike those two, this one has no linter behind it, so the note is the only signal available.

Does this PR introduce any user-facing change?

No. Documentation for contributors and agent tooling only.

How was this patch tested?

No tests; documentation only. Verified the added text is pure ASCII, per the rule in the paragraph immediately below it:

git diff -U0 -- AGENTS.md | grep '^+' | grep -v '^+++' | grep -P "[^\x00-\x7F]"

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

Generated-by: Cursor (Opus 5)

[szehon-ho]

@cloud-fan @HyukjinKwon do you have any thought on it? i sometimes find annoying the ai generate a private method and put it on top