engines node>=18 contradicts node:sqlite cost backend (needs >=22.5); doctor doesn't warn

[ils15]

Summary

package.json declares "engines": {"node": ">=18"} (package.json:118-120), but since B3-06 the pantheon_cost tool has a single backend: read-only node:sqlite, which only exists on Node >= 22.5. Users on Node 18–22 get UNSUPPORTED from pantheon_cost with no upfront warning, and doctor.mjs performs no Node version / node:sqlite availability check.

Evidence

• package.json:118-120 — engines.node >= 18
• CHANGELOG.md:30 + README.md:88-89 — single node:sqlite backend, UNSUPPORTED on Node < 22.5, CLI fallback removed (intentional, fail-closed)
• scripts/doctor.mjs — no Node version or node:sqlite check (validated: zero references)
• Live validation on a Node runtime without node:sqlite: pantheon_cost → UNSUPPORTED: No such built-in module: node:sqlite (honest, but nothing warned earlier in the install/doctor path)

Impact

• Misleading engines range: install succeeds on Node 18, cost tool silently unavailable
• Doctor reports 0 errors while a documented feature is dead on that runtime

Suggested fixes (non-breaking preferred)

• (a) doctor: read process.version; if < 22.5, emit a WARNING (not error): "pantheon_cost will report UNSUPPORTED — node:sqlite requires Node >= 22.5"
• (b) Optionally probe node:sqlite availability directly (more robust than version comparison)
• (c) Keep engines at >=18 if the rest of the plugin supports it, but document the cost-tool requirement in the README install section
• (d) Alternative (breaking): bump engines to >=22.5 — only if Node 18 support is not intentional