test(cli): reproduce updater executable orphaning

[opencode-agent]

Summary

Adds a deterministic subprocess regression reproducer for the executable-lifetime precondition behind #32002 and #32464.

The test installs a native executable through the same global Bun package-manager layout used by OpenCode, leaves it running, replaces the installed package, and verifies that:

• the original process remains alive
• its mapped executable changes from a named package path to the same path marked (deleted)

Current V2 calls global npm/pnpm/Bun/Yarn installation from its long-running background service updater every ten minutes, so this is the same inode-orphaning operation performed while OpenCode remains active.

On macOS 26, independent controlled reproductions correlate this orphaned mapping with repeated vn_getpath failed events and one unreclaimed 1 KiB data.kalloc.1024 allocation per failure. The exact OpenCode upgrade-under-load reproducer is https://gist.github.com/markjaquith/b6915a8f1b35939af04c166cd01b3b41.

Local proof

before: .../node_modules/opencode-updater-orphan-fixture/bin/opencode-updater-orphan-fixture
after:  .../node_modules/opencode-updater-orphan-fixture/bin/opencode-updater-orphan-fixture (deleted)
process: still running

Checks

• bun test test/updater-orphan.test.ts
• bun typecheck

Requested by: @kommander (Sebastian via Slack)