Tests: cover remaining runHarperCommand startup-watchdog branches

[heskew]

Summary

Follow-up from #8 / #9. The lifecycle race fixes added the repo's first tests (test/), covering the port helpers, the startup watchdog's main paths, and killHarper's signal logic. A few runHarperCommand branches remain uncovered and are cheap to test — the fixture-script harness in test/harperLifecycle.test.ts (spawning short .cjs scripts via harperBinPath) already supports them, so no real Harper is needed.

Branches to cover

• Completion marker split across stream chunks — a fixture that emits successfully started across two stdout.write calls/chunks should still be detected as ready (verifies the stdout.includes(...) accumulation, not just per-chunk matching).
• Spawn-error path — when the runtime/binary can't spawn (e.g. a non-existent/non-executable harperBinPath), runHarperCommand rejects and closes the stdout.log/stderr.log write streams (no leaked FDs). 'exit' never fires on a failed spawn.
• CI-aware absolute cap default — DEFAULT_STARTUP_MAX_MS resolves to 300s when process.env.CI is set and 120s otherwise (the IS_CI branch).
• Post-readiness log streaming — after the ready marker, continued process output is still written to the logDir files and is not re-accumulated into the returned startupOutput snapshot.

Notes

These are functional tests that spawn node child processes — same style as the existing test/ suite. Low effort; good first issue.

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