test: exclude test/** from coverage on every Node leg and ratchet thresholds to 99/96/98

[hivecommons-hive]

Test Improvement

npm test (scripts/run-tests.mjs) gates line/branch/function coverage on every CI leg (Node 22/24/26). Two problems surfaced while measuring:

1. The legs were gating different aggregates. Node 24+ excludes test files from the --experimental-test-coverage report by default; Node 22 does not. On Node 22 the report included test/*.test.js, so the functions gate (97) had a 0.2-point margin (97.20%) while on Node 26 it had 2.4 — a harmless test-only refactor could have gone red on Node 22 alone. This PR passes --test-coverage-exclude=test/** so every leg measures dist/ + scripts/ only.
2. Thresholds had not moved since [quality] pin subscribe SIGINT exit and add coverage-threshold script #71/test: enforce declared coverage thresholds when npm test runs on Node >= 22 [quality] #86 while coverage climbed. With all legs now measuring the same files (99.68 / 97.92 / 99.37 on both Node 22.23 and 26.10), the gate is ratcheted from 98/93/97 to 99/96/98.

A new runner test pins the thresholds CONTRIBUTING.md documents to the ones COVERAGE_FLAGS enforces, so the two cannot drift. CONTRIBUTING.md now describes npm test (Node ≥ 22) as the coverage gate, matching CI.

Deferred, not forgotten

package.json's test:coverage alias still carries the old flags (98/93/97, no exclude). #259 currently edits package.json and the hub refuses a second App-bot PR over the same file while it is open; the one-line alias update will follow once #259 lands. CI is unaffected — it runs the runner, not the alias.

Verification

• Node 26.10: npm test → 341 pass, all files 99.68 / 97.92 / 99.37, exit 0
• Node 22.23: node scripts/run-tests.mjs → 341 pass, all files 99.68 / 97.92 / 99.37, exit 0
• Node 22.23 without the exclude at the new thresholds (control): 95.84% branch coverage does not meet threshold of 96%, 97.20% function coverage does not meet threshold of 98%

No changelog.d fragment: test-infrastructure only, no user-visible change.

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Filed by quality agent (ACMM L4/L6 — full mode)

— hive: agent=quality backend=copilot model=claude-fable-5.1 copilot=1.0.88

[kubestellar-prow]

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