Chore: PowerToys extension development dev guide, and clean an unused msix declaration (#45967)

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## Summary of the Pull Request
PowerToys extension development dev guide, and clean an unused msix
declaration

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## PR Checklist

- [ ] Closes: #xxx
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issues) -->
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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## Detailed Description of the Pull Request / Additional comments

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## Validation Steps Performed

src/modules/cmdpal/doc/powertoys-extension-local-development.md

@@ -0,0 +1,47 @@
+# Local PowerToys Extension Development
+
+This guide is for iterating on `src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.PowerToys/Microsoft.CmdPal.Ext.PowerToys.csproj`.
+
+The extension is registered through the shared sparse package defined in `src/PackageIdentity/AppxManifest.xml`. That manifest declares `Microsoft.CmdPal.Ext.PowerToys.exe` at the sparse package root, so the sparse package and the extension must be built for the same platform and configuration, for example `x64\Debug`.
+
+## Local development loop
+
+1. Build `src/PackageIdentity/PackageIdentity.vcxproj`.
+
+   This creates `PowerToysSparse.msix` in the repo output root for the selected platform and configuration, and prints the `Add-AppxPackage` command you should run next.
+
+2. Trust the development certificate before running `Add-AppxPackage`.
+
+   The `PackageIdentity` build creates or reuses `src/PackageIdentity/.user/PowerToysSparse.certificate.sample.cer`.
+
+   Import it into `CurrentUser\TrustedPeople`:
+
+   ```powershell
+   $repoRoot = "C:/git/PowerToys"
+   Import-Certificate -FilePath "$repoRoot/src/PackageIdentity/.user/PowerToysSparse.certificate.sample.cer" -CertStoreLocation Cert:\CurrentUser\TrustedPeople
+   ```
+
+   If Windows still reports a trust failure such as `0x800B0109`, also import the same certificate into `Cert:\CurrentUser\TrustedRoot`.
+
+3. Run the `Add-AppxPackage` command printed by the `PackageIdentity` build.
+
+   That registers `Microsoft.PowerToys.SparseApp` as a sparse package and points it at the matching output root through `-ExternalLocation`.
+
+   The command will look like this:
+
+   ```powershell
+   Add-AppxPackage -Path "<repo>\<Platform>\<Configuration>\PowerToysSparse.msix" -ExternalLocation "<repo>\<Platform>\<Configuration>"
+   ```
+
+4. Build `src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.PowerToys/Microsoft.CmdPal.Ext.PowerToys.csproj` in the same platform and configuration.
+
+   This project writes `Microsoft.CmdPal.Ext.PowerToys.exe` directly into the sparse package root, such as `x64\Debug` or `ARM64\Debug`. That matches the `Executable="Microsoft.CmdPal.Ext.PowerToys.exe"` entry in `src/PackageIdentity/AppxManifest.xml`.
+
+5. Restart Command Palette.
+
+   Close any running CmdPal instance and launch it again so it reloads app extensions and picks up the rebuilt `Microsoft.CmdPal.Ext.PowerToys` binaries.
+
+## When to repeat each step
+
+- Rebuild and re-register `PackageIdentity` when the sparse package manifest changes, the signing certificate changes, or you switch to a different output root such as `ARM64\Debug`.
+- For normal code changes in `Microsoft.CmdPal.Ext.PowerToys`, rebuilding the extension project and restarting CmdPal is enough.

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.PowerToys/app.manifest

@@ -1,11 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
 <assembly manifestVersion="1.0" xmlns="urn:schemas-microsoft-com:asm.v1">
-  <assemblyIdentity version="1.0.0.0" name="PowerToysExtension.app" />
-  <msix xmlns="urn:schemas-microsoft-com:msix.v1"
-        packageName="Microsoft.PowerToys.SparseApp"
-        applicationId="PowerToys.CmdPalExtension"
-        publisher="CN=PowerToys Dev, O=PowerToys, L=Redmond, S=Washington, C=US" />
-
   <asmv3:application xmlns:asmv3="urn:schemas-microsoft-com:asm.v3">
     <asmv3:windowsSettings>
       <ws2:dpiAwareness xmlns:ws2="http://schemas.microsoft.com/SMI/2016/WindowsSettings">PerMonitorV2</ws2:dpiAwareness>