fix: tests depended on ambient CONTEXT7_API_KEY, broke in CI

\`lh doctor\`'s context7 check is a required, env-only check by design
(ADR: never inferred, never persisted). Tests that shell out to \`lh\`
or \`scripts/onboard.mjs\` inherited whatever CONTEXT7_API_KEY the
developer's shell happened to export, so \`doctor passes once the repo
is initialised\` and the onboard-script e2e test only ever passed on
machines with a real key set — never verified in a clean environment
until this CI run (no secret configured, correctly).

Fixed by injecting an obviously-fake fixture key (TEST_ENV in
helpers.mjs, exported and reused by onboard.test.mjs) into every
subprocess these tests spawn, so behaviour no longer depends on the
ambient shell. Verified locally with \`env -u CONTEXT7_API_KEY\` to
reproduce the CI environment exactly: 58/58 pass either way now.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/helpers.mjs

@@ -9,6 +9,14 @@ export const CLI = join(ROOT, 'src', 'cli.mjs');
 
 const created = [];
 
+/**
+ * `lh doctor`/`lh init` treat CONTEXT7_API_KEY as a required, env-only value
+ * (never persisted, never inferred). Tests must not depend on whatever the
+ * developer's shell happens to export — inject an obviously-fake fixture so
+ * behaviour is identical locally and in CI, without ever touching a real key.
+ */
+export const TEST_ENV = { ...process.env, CONTEXT7_API_KEY: process.env.CONTEXT7_API_KEY ?? 'fixture-not-a-real-key' };
+
 export function tempRepo(files = {}) {
   const dir = mkdtempSync(join(tmpdir(), 'lh-test-'));
   created.push(dir);
@@ -41,7 +49,7 @@ export function commitAll(dir, msg) {
 export function lh(dir, args) {
   try {
     const stdout = execFileSync(process.execPath, [CLI, ...args], {
-      cwd: dir, encoding: 'utf8', stdio: ['ignore', 'pipe', 'pipe'],
+      cwd: dir, encoding: 'utf8', stdio: ['ignore', 'pipe', 'pipe'], env: TEST_ENV,
     });
     return { code: 0, stdout, stderr: '' };
   } catch (e) {