docs: remove linter-owned rules from AGENTS.md

[myakove]

Problem

AGENTS.md duplicates rules already enforced by pre-commit tools. That wastes AI context and creates drift risk.

Root cause

• No Inline Imports is fully enforced by ruff PLC0415
• str = None → str | None = None is enforced by mypy no_implicit_optional
• Broader type-annotation guidance (Any justified, complete annotations) is only partially covered by mypy and must stay

Proposed approach

1. Drop the full No Inline Imports (MUST) section from AGENTS.md; replace with a short one-liner pointing to ruff PLC0415 (and note TYPE_CHECKING exception if still useful)
2. Delete only the sentence under Type Annotations about default None / str | None
3. Keep Type Annotations MUST for complete annotations + justified Any
4. Update any cross-references to "No Inline Imports" (e.g. conftest placement section)

Out of scope

• Security-related hooks/rules
• Adding new ruff rules (UP006/UP035) — separate follow-up
• Automating other AGENTS.md rules

Done

• No Inline Imports full section removed or reduced to PLC0415 one-liner
• str = None sentence removed from Type Annotations
• Complete annotations + justified Any guidance retained
• Cross-references to "No Inline Imports" updated
• Pre-commit passes