Run each test file in its own process: the parent runner drops file tails under CI load

[maxff77]

Problem

CI on main (after #167) is red with TEST GATE: FAIL [SHORT_RUN]: 1035 of 1074 tests, fail 0, exit 0, on 3 of 3 attempts. The tree is whole — the same commit reports 1074/128 locally and on an idle 4-core Linux VPS (Node 24.21). Each CI attempt lost the tail of different files (e.g. R3–R12 of anthropic-native-parity, the end of thinking-retention).

Cause: node --test --test-force-exit tests/*.test.js — the parent runner with one child per file — loses the end of a child's piped output under CPU load. Reproduced on the VPS by adding six CPU hogs (1069, then 1074 on retry). It is load-dependent, and GitHub's 2-vCPU runner is always loaded, so the gate's retries could never save it. (This is the same drop the gate was written to detect; it just cannot be retried around on CI.)

Fix

• tools/test-gate.js — stop using the parent runner. Every file runs as node --test --test-isolation=none --test-force-exit <file>; the gate reads each process's stdout to EOF before counting anything and sums the per-file summaries (sumSummaries: a file with no summary voids the attempt as NO_SUMMARY, never subtracted quietly). Bounded pool (TEST_GATE_CONCURRENCY, default min(4, cores-1)), one watchdog per attempt. --test-force-exit stays: 26 files never exit without it.
• .github/workflows/ci.yml — 300 s per attempt, 2 attempts (measured: ~10 s local, 77 s on the 4-core VPS at concurrency 1).
• tests/tool-prompt.test.js (P14) — threshold 400 → 1500 ms. Measured 50 ms on a Mac, 210–225 ms on the VPS idle, 445 ms under load; the quadratic regression it guards was ~2 s, so 1500 still catches it and stops flaking on a shared runner.
• tests/test-count-gate.test.js — three tests for sumSummaries. Baseline 1074 → 1077.

Verification

• Local: npm test → PASS 1077/128; eslint clean.
• Linux VPS, this exact tree, six CPU hogs, single attempt: PASS 1077/128 twice (65 s, 68 s). The previous gate under the same load produced a silent short pass.