chore: remove remaining VP_HOME usage (especially snapshot tests)

[forehalo]

Context

VpDirs resolution still honors deprecated VP_HOME as a full legacy-root pin for compatibility, but much of the repo still sets or assumes VP_HOME as the primary install root. That fights the split layout (VP_*_DIR / XDG_* / platform defaults) and keeps fixtures on a monolithic shape.

Follow-up to #2346 / #827.

Goal

Stop using VP_HOME except where it is required as a read-only deprecated override (custom roots). Prefer VP_BIN_DIR / VP_DATA_DIR / VP_CACHE_DIR and XDG/platform defaults everywhere else.

Scope (high signal)

• PTY snapshot suite (crates/vp_cli_snapshots/): provision per-case homes with split dirs or explicit VP_*_DIR, not a permanent VP_HOME=~/.vite-plus baseline; update fixtures that expand $VP_HOME/...
• CI / bootstrap: pnpm bootstrap-cli, GITHUB_PATH entries, scripts that export VP_HOME only to force legacy shape
• Docs / comments / help strings that still describe VP_HOME as the default home
• Tests that seed ~/.vite-plus solely to trigger grandfathering (replace with split targets or explicit VP_*_DIR)
• Audit remaining set_var("VP_HOME") / VP_HOME: in Rust, TS, shell, and workflow YAML

Non-goals

• Removing the deprecated read of VP_HOME in VpDirs resolution (keep until a later deprecation cut)
• Auto-migrating on-disk ~/.vite-plus trees (tracked separately)

Acceptance

• Snapshot runner and fixtures do not require VP_HOME for the default happy path
• New tests document layout via VP_*_DIR / XDG / platform defaults
• Grep for product code that sets VP_HOME is limited to documented custom-root / trampoline edge cases