chore: review fix/agents guidance import lint

[crowecawcaw]

What was the problem/requirement? (What/Why)

Agents in this codebase often put imports in functions and write long verbose comments.

What was the solution? (How)

Add a linter for imports not at the top of the file
Add AGENTS.md guidance about comments

What is the impact of this change?

Agents produce better code with less feedback.

How was this change tested?

CI

Was this change documented?

n/a

Does this PR introduce new dependencies?

No

Is this a breaking change?

No

Does this change impact security?

No

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

Testing

Manually verified the new lint config locally (ruff 0.15.22 via hatch run lint, macOS):

1. Rules actually fire: created a scratch file with a module-level import after executable code (no noqa) and an unsorted __all__. hatch run ruff check flagged both — E402 Module level import not at top of file and RUF022 `__all__` is not sorted — and exited non-zero.
2. Escape hatch works: adding # noqa: E402 to the late import made the file pass, matching the existing suppressions in the tree (e.g. src/deadline/client/cli/_common.py, test/ui/conftest.py, test/blender_submitter_ui/* — deferred/bpy-gated imports).
3. No spurious hits on the real tree: after removing the scratch file, hatch run lint passes clean on the full repo (ruff check . — all checks passed; ruff format --check — 328 files already formatted; mypy — no issues in 311 files).