feat(windows): scaffolding pass for Windows support

First-pass changes to let pkgm at least start on Windows so CI can
surface real gaps instead of failing at "/usr/local doesn't exist":

- standardPath(): add Windows case returning the baseline System32 /
  System32\Wbem dirs joined with `;`. Introduces PATH_SEP module
  constant used wherever we tokenise $PATH (a literal `:` split would
  shred `C:\` drive prefixes on Windows).
- install_prefix(): on Windows, skip the /usr/local probe and return
  Path.home()/.local. System-wide installs under %ProgramFiles% need
  UAC modelling — out of scope here.
- get_pkgx(): use PATH_SEP and look for `pkgx.exe` on Windows.
- install() PATH-contains check: use PATH_SEP.
- Stub writing: skip the Deno.chmod(0o755) on Windows (it throws).
  The stub content is still POSIX shell — proper `.cmd`/`.ps1` stub
  emission is a follow-up.
- CI: add windows-latest to the `test` matrix with
  `strategy.fail-fast: false`. The existing `continue-on-error: true`
  keeps the matrix overall green; the Windows leg surfaces what's
  still broken so we can iterate.

Open design questions deliberately left for the draft-PR description:
- System-wide install prefix on Windows (%ProgramFiles%? user-only?)
- Elevation model (UAC? refuse?)
- Stub format (.cmd, .ps1, or compiled wrapper?)
- libpkgx + pkgxdev/setup Windows-readiness verification

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: tannevaled

.github/workflows/ci.yml

@@ -27,10 +27,16 @@ jobs:
   test:
     continue-on-error: true
     strategy:
+      fail-fast: false
       matrix:
         os:
           - macos-latest
           - ubuntu-latest
+          # Windows: scaffolding pass — steps below rely on shebang
+          # interpretation and POSIX paths, so most will fail until
+          # pkgm grows native Windows install/stub support. See the
+          # draft PR description for the open design questions.
+          - windows-latest
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4

pkgm.ts

@@ -13,7 +13,23 @@ import { ensureDir, existsSync, walk } from "jsr:@std/fs@^1";
 import { parseArgs } from "jsr:@std/cli@^1";
 const { hydrate } = plumbing;
 
+// Path-separator: `;` on Windows, `:` on POSIX. Used everywhere we
+// split or join $PATH, since C:\ on Windows would tokenise wrong with `:`.
+const PATH_SEP = Deno.build.os == "windows" ? ";" : ":";
+
 function standardPath() {
+  if (Deno.build.os == "windows") {
+    // Windows: no /usr/local hierarchy and no homebrew. Return the
+    // baseline system dirs so subprocesses can still locate built-in
+    // commands (cmd, find, etc.) when we feed them an explicit PATH.
+    const systemRoot = Deno.env.get("SystemRoot") ?? "C:\\Windows";
+    return [
+      `${systemRoot}\\System32`,
+      systemRoot,
+      `${systemRoot}\\System32\\Wbem`,
+    ].join(PATH_SEP);
+  }
+
   let path = "/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin";
 
   // for pkgx installed via homebrew
@@ -194,7 +210,12 @@ async function install(args: string[], basePath: string) {
 
         await Deno.remove(to_stub); //FIXME inefficient to symlink for no reason
         await Deno.writeTextFile(to_stub, sh.trim() + "\n");
-        await Deno.chmod(to_stub, 0o755);
+        // Windows has no POSIX exec bit; Deno.chmod throws there. Stub
+        // content is still POSIX shell at this point — proper `.cmd`/`.ps1`
+        // stub emission is a separate TODO (see draft PR description).
+        if (Deno.build.os != "windows") {
+          await Deno.chmod(to_stub, 0o755);
+        }
 
         rv.push(to_stub);
       }
@@ -204,7 +225,7 @@ async function install(args: string[], basePath: string) {
   if (
     !Deno.env
       .get("PATH")
-      ?.split(":")
+      ?.split(PATH_SEP)
       ?.includes(new Path(basePath).join("bin").string)
   ) {
     console.error(
@@ -518,8 +539,9 @@ function symlink_with_overwrite(src: string, dst: string) {
 }
 
 function get_pkgx() {
-  for (const path of Deno.env.get("PATH")!.split(":")) {
-    const pkgx = join(path, "pkgx");
+  const exe = Deno.build.os == "windows" ? "pkgx.exe" : "pkgx";
+  for (const path of Deno.env.get("PATH")!.split(PATH_SEP)) {
+    const pkgx = join(path, exe);
     if (existsSync(pkgx)) {
       const out = new Deno.Command(pkgx, { args: ["--version"] }).outputSync();
       const stdout = new TextDecoder().decode(out.stdout);
@@ -758,6 +780,14 @@ async function update() {
 }
 
 function install_prefix() {
+  // Windows has no /usr/local analogue; the per-user prefix is the
+  // only sensible default for now. System-wide installs on Windows
+  // (e.g. under %ProgramFiles%) require UAC elevation modelling and
+  // are out of scope for the initial port — see the draft PR
+  // description for the design questions.
+  if (Deno.build.os == "windows") {
+    return Path.home().join(".local");
+  }
   // if /usr/local is writable, use that
   if (writable("/usr/local")) {
     return new Path("/usr/local");