Support cache config for built-in vp build and vp pack

https://github.com/voidzero-dev/vite-plus/pull/1992#discussion_r3503183576

## Problem

Today, Vite Task cache applies when a build is run as a task, for example `vpr build` or `vp run build`. A user can get cache hits from that task path, but direct built-in commands like `vp build` and `vp pack` do not use the task cache.

That makes the behavior hard to explain. `vpr build` can hit cache while `vp build` always runs, even though both may execute the same build command.

## Possible direction

Add a config option that lets projects enable caching for built-in `vp build` and `vp pack` commands. This would make the first-party command path and the task-runner path behave more consistently.

Start by comparing the built-in `vp build` and `vp pack` command paths with task-based `vpr build` and `vp run build`, then read the cache behavior discussed in PR 1992. Identify where project configuration is loaded and where task caching is applied. Done means a documented config option enables cache hits for both built-in commands and makes their behavior consistent with the task paths.

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