feat(env): make vp env off disable Node.js management for all vp commands

[fengmk2]

Previously, vp env off only affected shim dispatch (node/npm/npx
invoked directly). All vp commands (create, build, install, etc.)
still downloaded managed Node.js via JsExecutor, ignoring the
system-first mode setting.

Now when vp env off is active, JsExecutor checks ShimMode before
downloading and uses the system-installed Node.js found in PATH.
This fixes NixOS/Guix (where downloaded binaries fail), air-gapped
environments, and users managing Node.js via other tools (mise, nvm).

• Add JsRuntime::from_system() constructor for system binary paths
• Add system-first checks to ensure_cli_runtime/ensure_project_runtime
• Expose find_system_tool as pub(crate) and deduplicate doctor.rs copy
• Update vp env off/on messaging to reflect broader scope
• Add CI E2E tests for system-first mode on all platforms

Closes #977

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[fengmk2]

• fix(migration): add vitest to devDependencies for peer dep resolution #1261
• feat(env): make vp env off disable Node.js management for all vp commands #1255 👈 (View in Graphite)
• main

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[cpojer]

Question regarding vp env off's expected behavior. I'm trying to figure out if we need three different modes here. Does it still shim node.js inside of a project that uses Vite+?

I imagine that there might be use-cases for three things:

• Vite+ does not manage Node.js → Always use system Node.js
• Vite+ only manages Node.js inside of projects that use vite-plus → otherwise fall back to system Node.js
• Vite+ always manages Node.js

Can you walk me through what our plan is exactly?

[fengmk2]

Before this PR change:
vp env off only disables the interception when executing node, npm and other shims, which will fall back to the system version of Node.js.
For any commands run through the global vp cli within projects that depend on vite-plus, the node version used in the process will point to the version declared in .node-version (because this functionality existed before the vp env command).

After this PR change:
vp env off will disable all node shims, including the logic within projects that depend on vite-plus, all falling back to the system version of Node.js.

I think after the change there are only 2 modes: either intercept everything or fall back everything, without the current intermediate state.

As for falling back to vp management mode when system Node.js is not found, I think this is reasonable because we assume some new users will only install vp without installing Node.js first.