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.

internal/wrapper/tools_test.go

@@ -43,3 +43,21 @@ func TestEveryToolHasAnExeName(t *testing.T) {
 		}
 	}
 }
+
+func TestIsTool(t *testing.T) {
+	for name := range Tools {
+		if !IsTool(name) {
+			t.Errorf("IsTool(%q) = false, want true (it's in Tools)", name)
+		}
+	}
+	for name := range nativeTools {
+		if !IsTool(name) {
+			t.Errorf("IsTool(%q) = false, want true (it's in nativeTools)", name)
+		}
+	}
+	for _, name := range []string{"download", "install", "env", "version", "help", "completion", "frobnicate", ""} {
+		if IsTool(name) {
+			t.Errorf("IsTool(%q) = true, want false", name)
+		}
+	}
+}