Output schemas: use a linear-time regex engine for pattern/patternProperties #131

From the review on #129 (deferred). OutputSchema.compile passes schemas to Ajv, which compiles pattern and patternProperties into native backtracking RegExp, so a pathological pattern can stall validation of model output. The schema comes from the invoking user or workflow author, not the model, and executor runs are timeout-bounded, so the impact is a self-inflicted stalled run.

Option: Ajv's code.regExp hook with an RE2 binding (check single-binary compatibility). The executor-side checker (aictrl-output.cjs) must make the same choice so verdict parity holds.