fix(ci): packed schema verification, native CLI exec, Node 26 baseline

[usehoplite]

Stacked on #7 — hardening from its final review pass plus two follow-up fixes.

What this fixes

CI never verified the packed schema (the headline feature of #7). The packed-artifact step imported dist/tui.js and asserted the entrypoint id, but nothing checked that options.schema.json made it into the tarball or that the opencode2-tps/options.schema.json export resolves from an installed copy. tests/options-schema.test.ts reads the repo file, so a files:/exports regression would have shipped a broken schema export with everything green. The step now asserts the tarball contains package/options.schema.json and imports the export through the package's exports map (with the required with { type: "json" } attribute).

scripts/check-compatibility.mjs was cwd-dependent and shell-dependent. It resolved node_modules/.bin against process.cwd() (breaking node scripts/check-compatibility.mjs from any other directory) and used npm's .bin wrapper, whose .cmd shim on Windows cannot be execFileSync'd without a shell. It now resolves against the script's location and executes @opencode/cli's real declared bin target (bin/opencode2.exe) — the native binary runs unshelled on every platform despite the .exe name, and both a normal install's postinstall and CI's existing --ignore-scripts prepare step put it there. The process.platform branch is gone.

CI ran Node 24 below the toolchain's documented floor. @opentui/core (0.5.11, what ^0.5.10 resolves to upstream) declares engines: { bun: ">=1.3.0", node: ">=26.4.0" }. CI's Node-side work (npm ci, tsc, oxlint, scripts/*.mjs, the packed-artifact check) moves to node-version: 26.4.0, and @types/node follows to ^26.0.0 (lock regenerated; resolves 26.6.1) so typechecking matches what CI actually runs. Node 26's setInterval typing adds a conditional rest-args overload that no two-parameter test double can satisfy, so the two timer doubles in tests/plugin.test.ts and tests/entrypoint.test.tsx now assert to typeof globalThis.setInterval at the assignment boundary — behavior unchanged.

Verification

All run locally under the actual CI baseline (Node 26.4.0 via mise, npm 11.17):

• lint: 0 warnings, 0 errors; tsc --noEmit clean with @types/node 26.6.1.
• bun test: 68 pass, 0 fail.
• scripts/build.mjs produces dist; check-compatibility passes from the repo root and from a foreign cwd, executing the native bin/opencode2.exe.
• Full pack cycle re-verified under Node 26.4.0 with the dev-resolved peers (@opentui/solid@0.5.11, solid-js@1.9.12): tarball contains package/options.schema.json, dist/tui.js imports with id === "opencode2.tps", and opencode2-tps/options.schema.json resolves through the exports map.
• No runtime source changes — src/ is untouched, so refactor: adopt src/tests/scripts layout and ship options.schema.json #7's verified behavior (including live hot-reload from src/, confirmed against a real opencode2 TUI) is unaffected.

Merge into #7 before merging #7 to main; when #7 lands and its branch is deleted, GitHub retargets this to main automatically and the diff collapses to just these commits.