fix(pi): spawn bundled Pi CLI directly under Node, not via bun

Follow-up to 2707370. That commit resolved the bundled
@earendil-works/pi-coding-agent/dist/cli.js path correctly but then
spawned it as "bun <cli.js>" on the assumption that the OS couldn't
exec a .js file directly.

Pi is a Node app (its package.json bin entry points at dist/cli.js
which carries "#!/usr/bin/env node"), and npm sets the exec bit
during install. The OS handles the shebang and runs the script under
Node with no extra runtime needed.

Requiring "bun" would have broken any Pi user who installed Pi
globally via `npm install -g @earendil-works/pi-coding-agent` without
also installing Bun. The subagent spawn would have failed with ENOENT
once the plugin used the bundled path.

Fix: drop the cliViaBun flag and bun wrapping entirely. Spawn the
resolved cli.js path directly. The OS resolves the shebang and runs
it under Node. Explicit piBinary override path was already direct-
spawn and is unaffected.

Verification:
- 297/297 Pi unit tests pass
- 25/25 subagent-runner tests pass
- 2/2 Pi historian e2e tests pass in 5s (pi-historian-success,
  pi-slow-historian) which are the tests that actually spawn Pi
  via the bundled resolution path
- Typecheck clean