feat(env)!: manage package manager environments

[liangmiQwQ]

This PR extends vp env to manage Node.js and package managers as peer environment components.

It adds package-manager defaults, session overrides, project pins, installation lifecycle, diagnostics, listings, and scoped environment commands. Package-manager selection keeps project declarations and lockfile detection in the resolution order, while unconfigured named shims use a cached latest release.

Command	Behavior
vp env	Shows the unified environment help.
vp env current [scope]	Shows the resolved Node.js runtime and selected package manager, including source, installation state, tool paths, and mode. Supports --json.
vp env print [scope]	Prints a shell snippet that prepends the resolved Node.js and package-manager bin directories to PATH.
vp env default [specs]	Shows both global defaults with no arguments. Bare versions set Node.js; qualified specs such as pnpm@10 set the package-manager fallback. --unset clears selected defaults.
vp env on [scope]	Enables managed mode for both components by default, or only Node.js/package managers when scoped.
vp env off [scope]	Enables system-first mode for both components by default, or only Node.js/package managers when scoped.
vp env setup	Creates or refreshes the Vite+ shell setup and shims for Node.js, npm, pnpm, Yarn, Bun, and their aliases.
vp env doctor [scope]	Diagnoses configuration conflicts, missing installs, shim state, and environment selection.
vp env which <tool>	Prints the executable path that the environment shim would select for a tool.
vp env pin [specs]	Shows or writes project pins. Bare versions target Node.js; qualified specs target a package manager. Existing compatible fields are preserved, and versions are installed unless --no-install is used.
vp env unpin [scope]	Removes effective project pins for both components by default, or only the requested component/source.
vp env list [scope]	Lists locally installed Node.js and package-manager versions in component sections. Supports --json.
vp env list-remote [scope] [pattern]	Lists registry versions for Node.js and package managers, with component filtering, LTS filtering, full history, sorting, and JSON output.
vp env exec [overrides] <command>	Runs a command in the resolved full environment. --node and --package-manager override individual components; run remains an alias.
vp env install [requests]	Installs the resolved full environment by default, a selected component, or explicit Node.js/package-manager specs.
vp env uninstall <specs>	Removes explicitly selected installed Node.js or package-manager versions.
vp env clean [scope]	Removes unused managed runtimes and package-manager installs while preserving active and configured-default versions.
vp env use [requests]	Activates the resolved or explicit environment for the current shell session, auto-installing by default. --unset clears session overrides.

This is currently opened against main as a draft while the stacked diff is being organized.

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