Run wrapped tools directly as vintner <tool> ..., no PATH needed

`vintner cl ...`, `vintner msbuild ...`, etc. now work without adding
<dest>/bin/<arch> to PATH or relying on the same-directory symlinks
`install` sets up there. wrapper.Run gained an explicit binDir
parameter (empty string preserves the existing os.Executable()-based
self-location for the ordinary multi-call/symlink case) so
cmd/vintner's new runTool can point it at a resolved toolchain
directory instead.

Resolution order: VINTNER_BIN if set (same meaning as `env --bin` -
point it at a <dest>/bin/<arch> directory directly, for a non-default
--dest or a specific architecture), else <defaultToolchainDir>/bin/
<hostArch> - the layout a plain `vintner download && vintner install`
with no --dest override produces. A missing toolchain gets a clear
error pointing at both fixes, rather than bubbling up whatever
wineenv.Load's env.json error looks like.

Also fixed shell completion falling out of sync with the actual tool
list: the bash/zsh scripts previously hand-copied tool/flag names
(and had already gone stale once - --with-dxsdk was missing from the
download flag completions since it was added). The tool name list is
now generated from wrapper.ToolNames() instead of hand-maintained,
and both scripts now complete tool names too, so `vintner <TAB>`
suggests `cl`, `link`, `msbuild`, etc. alongside the management
subcommands.

Verified end-to-end with a minimal PATH (/usr/bin:/bin only, no
toolchain dir on it at all): `vintner cl /nologo hello.c` compiled
successfully, and `vintner msbuild -t:Rebuild ...` rebuilt the same
real KMDF driver verified earlier this session - both via the default
~/.vintner/bin/<hostArch> resolution, no VINTNER_BIN override needed.

README.md

@@ -21,6 +21,7 @@ approach: download the real MSVC/WinSDK, wrap the compiler under Wine.
 - [Installation](#installation)
 - [Quick start](#quick-start)
 - [Commands](#commands)
+- [Invoking tools without PATH](#invoking-tools-without-path)
 - [Building drivers (WDK)](#building-drivers-wdk)
 - [Building against D3DX9 (DirectX SDK)](#building-against-d3dx9-directx-sdk)
 - [Automated/scripted builds](#automatedscripted-builds)
@@ -102,6 +103,9 @@ export PATH=~/.vintner/bin/x64:$PATH
 cl /nologo /EHsc hello.cpp
 ```
 
+Don't want to touch PATH? Skip step 3 and run tools through vintner
+directly instead - see [Invoking tools without PATH](#invoking-tools-without-path).
+
 ## Commands
 
 ```
@@ -132,6 +136,24 @@ package id or through `--with-*`. `--print-deps-tree` prints the
 dependency tree of whatever would actually be selected — honoring every
 other flag — without downloading anything.
 
+## Invoking tools without PATH
+
+`cl`, `link`, `msbuild`, and the rest also work as `vintner <tool>
+[args...]`, with no need to add `<dest>/bin/<arch>` to `PATH` or rely on
+the symlinks `install` sets up there:
+
+```bash
+vintner cl /nologo /EHsc hello.cpp
+vintner msbuild MyProject.sln
+```
+
+Resolves the toolchain to use from `VINTNER_BIN` if set (same meaning as
+`env --bin`: point it at a `<dest>/bin/<arch>` directory directly — useful
+for a non-default `--dest`, or to pick a specific architecture when more
+than one is installed), otherwise defaults to
+`~/.vintner/bin/<host-arch>`, the layout a plain `vintner download &&
+vintner install` with no `--dest` override produces.
+
 ## Building drivers (WDK)
 
 `--with-wdk` also fetches the Windows Driver Kit: headers, import libs,