test-audio-glitch

Agents regression surface (#16070)

* AGENTS.md: require a regression-surface check before a change is done

The minimal-invasiveness rules say what to prefer; nothing made an
agent check the final diff against them, so a feature could still route
the old path through its new code while every principle was "followed".
This adds the gate: audit every modified existing path, keep feature-off
on the old code, report the regression surface, and treat an
unnecessarily rewritten legacy path as a review finding whatever the
tests say.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EZ49AbZJYfm8NTp5yDPMab

* AGENTS.md: a scope check before shared code is touched

The minimal-invasiveness rules are principles; this adds the stop
condition that makes them mechanical. A fix for one path stays in that
path, and an unrelated caller needing a placeholder argument to satisfy
a changed signature is the signal that it did not.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EZ49AbZJYfm8NTp5yDPMab

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Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>