Shorten stdio test comments

[Kludex]

Summary

• Shorten stdio test comments and docstrings while retaining non-obvious constraints.
• Instruct coding agents to keep comments brief and avoid narrating code.

Validation

• uv run --frozen pre-commit run --all-files
• uv run --frozen pytest -q tests/interaction/transports/test_stdio.py tests/transports/stdio/test_lifecycle.py tests/transports/stdio/test_posix.py tests/transports/stdio/test_windows.py

AI Disclaimer

This PR was developed with the assistance of either Claude or Codex. I've reviewed and verified the changes.

[cubic-dev-ai]

No issues found across 6 files

_{Re-trigger cubic}

[claude]

LGTM — comment and docstring trimming only, and the follow-up commit resolves both points from my earlier review.

What was reviewed:

• Confirmed every changed line in the five test files is a comment or docstring; no code, assertions, timeout values, or env handling changed.
• Checked that the load-bearing pragmas (no branch, lax no cover, no cover) all survive the rewording in place.
• Verified the AGENTS.md change only adds a comment-brevity guideline and touches no other rules.

Extended reasoning...

Overview

This PR trims verbose comments and docstrings in the stdio transport test suite (tests/interaction/transports/_stdio_server.py, tests/interaction/transports/test_stdio.py, and the three files under tests/transports/stdio/) and adds a two-line "keep comments brief" bullet to AGENTS.md's Code Quality section. The diff removes ~200 lines of prose and replaces long explanations with one-line summaries. I read the full diff line by line: every added/removed line is inside a comment or docstring — no executable statement, assertion, timeout bound, pragma directive, or environment-variable handling changed.

Security risks

None. No production code under src/ is touched, and the test changes are non-executable. The AGENTS.md addition is a documentation guideline with no security implications.

Level of scrutiny

Low scrutiny is appropriate for a comments-only change, with two specific checks warranted: (1) that the trimming did not accidentally delete a coverage pragma or alter code adjacent to reworded comments — I verified the # pragma: no branch, # pragma: lax no cover, and # pragma: no cover markers all remain attached to the same lines; (2) that the shortened comments do not lose or misstate load-bearing constraints. My prior review flagged two such cases, and the follow-up commit cf29fb2 fixed both: the cov.stop() comment now records that it leaves nothing for coverage's atexit hook to rewrite during interrupted teardown, and the warning-suppression comment now correctly attributes the SyntaxWarning to Python 3.14 rather than "AnyIO 3.14".

Other factors

The bug hunt exited on dry_streak with no findings, and this run surfaced nothing new. The only third-party event is a COMMENTED (not CHANGES_REQUESTED) review from another automated tool, which does not block approval. The author is the repository maintainer, and the intent — enforcing the new comment-brevity guideline on its worst offenders — is coherent between the AGENTS.md addition and the test edits. Since the change cannot alter runtime behavior and my earlier feedback was demonstrably addressed in the code, approval is warranted.