docs: record Askr North Star

AGENTS.md

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+# AGENTS.md
+
+Operational guide for `@askrjs/testing`, which owns transport-neutral request
+injection and focused browser-harness guidance.
+
+## Askr North Star
+
+Keep every helper's simulation boundary explicit so a reader knows which real
+mechanisms it does and does not exercise. Reject invalid requests, redirect
+state, lifecycle use, and harness configuration with actionable errors. Test
+redirect, abort, streaming, cleanup, and thrown paths. Keep request injection,
+browser testing, and real transport testing as visible, non-interchangeable
+seams. Prefer explicit handlers and requests over hidden globals or matcher
+magic. Add helpers only for demonstrated application test needs.
+
+Run `npm run check` before declaring a change ready.
+
+## Optimization Gate
+
+A benchmark number is only half of an optimization's success criterion. The
+change must also preserve a causal path that a human or agent can narrate in one
+sentence.
+
+Every benchmark-driven change must include:
+
+1. the one-sentence causal description of the optimized path;
+2. the exact fallback trigger and proof that optimized and fallback paths have
+   identical observable behavior and error surfaces;
+3. an explicit legibility-cost statement, including `none` when no new path or
+   concept is introduced; and
+4. evidence that a measured bottleneck in a real application justifies the
+   optimization now.
+
+Prefer making the existing single path faster. New caches, inference,
+memoization, shortcuts, fast paths, or scheduler states require an explicit
+legibility decision; a speedup alone does not justify them.