skills(writing-user-outputs): document the stderr half of the anstream macro rule

[worktrunk-bot]

Skill drift. writing-user-outputs is the thing loaded before editing anything that produces user-visible strings, and its "Printing output" section states only the stdout half of a two-sided rule — which println! is in scope, and the BrokenPipe panic that turns on it. The stderr half is absent, even though ec0f3a3 (#3771) made it structural: check_stderr_macros_come_from_styling now requires every bare eprint! / eprintln! under src/ to resolve to anstream's.

The gap is exactly the mistake #3771 fixed in five files. A developer following the skill's own example import — use worktrunk::styling::{eprintln, println, stderr}; — and then reaching for eprint! mid-block gets std's macro, which keeps ANSI on a redirected stderr where the eprintln! two lines up drops it. The skill never names eprint! at all, and #3771's commit message notes the suite cannot catch this: CLICOLOR_FORCE=1 forces color on both printers, so every snapshot agrees whichever macro is in scope.

One paragraph, placed directly after the println! one it mirrors: the stripping asymmetry and its user-visible symptom, that eprint! is the half that slips and why, the two ways to satisfy the rule (import, or qualify as styling::eprintln!(…)), the guard test that enforces it, and that STD_STDERR_ALLOWED_PATHS exempts whole files rather than calls.

No regression test — the change is skill prose. The behavior it describes is already pinned by check_stderr_macros_come_from_styling and test_stderr_narration_strips_ansi_when_piped; what was missing is that a developer reads the rule before tripping the guard rather than after.