feat(winmd-api-search): add WinMD cache generator script and related files (#45606)

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fix/what does it change/how was it tested (even manually, if necessary)?
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## Summary of the Pull Request

Adds a new Copilot agent skill (`winmd-api-search`) that lets AI agents
discover and explore Windows desktop APIs by searching a local cache of
WinMD metadata. The skill covers Windows Platform SDK, WinAppSDK/WinUI,
NuGet package WinMDs, and project-output WinMDs — providing full API
surface details (types, members, enumeration values, namespaces) without
needing external documentation lookups.

**Key components:**

- `.github/skills/winmd-api-search/SKILL.md` — Skill definition with
usage instructions, search/detail workflows, and scoring guidance
- `scripts/Invoke-WinMdQuery.ps1` — PowerShell query engine supporting
actions: `search`, `type`, `members`, `enums`, `namespaces`, `stats`,
`projects`
- `scripts/Update-WinMdCache.ps1` — Orchestrator that builds the C#
cache generator, discovers project files, and runs the generator
- `scripts/cache-generator/CacheGenerator.csproj` + `Program.cs` — .NET
console app using `System.Reflection.Metadata` to parse WinMD files from
NuGet packages, project references, Windows SDK, and packages.config
into per-package JSON caches
- `scripts/cache-generator/Directory.Build.props`,
`Directory.Build.targets`, `Directory.Packages.props` — Empty isolation
files to prevent repo-level Central Package Management and build targets
from interfering with this standalone tool

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## PR Checklist

- [ ] Closes: #xxx <!-- Replace with issue number if applicable -->
- [x] **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 — N/A: This is an offline
agent skill (PowerShell + standalone .NET tool) with no integration into
the main product build or runtime. Validated manually by running the
cache generator across multiple project contexts (ColorPickerUI,
CmdPal.UI, runner, ImageResizer, etc.) and exercising all query actions.
- [ ] **Localization:** All end-user-facing strings can be localized —
N/A: No end-user-facing strings; this is an internal developer/agent
tool
- [ ] **Dev docs:** Added/updated — The SKILL.md itself serves as the
documentation
- [ ] **New binaries:** Added on the required places — N/A: The cache
generator is a standalone dev-time tool, not shipped in the installer
- [ ] **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: N/A

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or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

### Cache Generator (`Program.cs`, ~1000 lines)

A self-contained .NET console app that:

1. **Discovers WinMD sources** from four channels:
   - `project.assets.json` (PackageReference — modern .csproj/.vcxproj)
   - `packages.config` (legacy NuGet format)
- `<ProjectReference>` bin/ output (class libraries producing `.winmd`)
   - Windows SDK `UnionMetadata/` (highest installed version)

2. **Parses WinMD files** using `System.Reflection.Metadata` /
`PEReader` to extract:
- Types (classes, structs, interfaces, enums, delegates) with full
namespace
- Members (methods with decoded signatures/parameters, properties with
accessors, events)
   - Enum values
   - Base types and type kinds

3. **Outputs per-package JSON** under `Generated Files/winmd-cache/`:
- `packages/<Id>/<Version>/meta.json` — package summary
(type/member/namespace counts)
   - `packages/<Id>/<Version>/namespaces.json` — ordered namespace list
- `packages/<Id>/<Version>/types/<Namespace>.json` — full type detail
per namespace
- `projects/<ProjectName>.json` — maps each project to its package set

4. **Deduplicates** at the package level — if a package+version is
already cached, it's skipped on subsequent runs.

### Build Isolation

Three empty MSBuild files (`Directory.Build.props`,
`Directory.Build.targets`, `Directory.Packages.props`) in the
cache-generator folder prevent the repo's Central Package Management and
shared build configuration from interfering with this standalone tool.

### Query Engine (`Invoke-WinMdQuery.ps1`)

Supports seven actions: `search` (fuzzy text search across
types/members), `type` (full detail for a specific type), `members`
(filtered members of a type), `enums` (enumeration values), `namespaces`
(list all namespaces), `stats` (cache statistics), and `projects` (list
cached projects with their packages).

<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

1. **Cache generation:** Ran `Update-WinMdCache.ps1` across 310+ project
files in the repo — 8 packages parsed, 316 reused from cache, all
completed without errors
2. **Query testing on multiple projects:**
   - `ColorPickerUI` — verified Windows SDK baseline (7,023 types)
- `Microsoft.CmdPal.UI.ViewModels` (after restore) — verified 13
packages, 49,799 types, 112,131 members including WinAppSDK,
AdaptiveCards, CsWinRT, Win32Metadata
   - `runner` (C++ vcxproj) — verified packages.config fallback path
   - `ImageResizerExt` — verified project reference WinMD discovery
3. **All seven query actions validated:** `stats`, `search`,
`namespaces`, `type`, `enums`, `members`, `projects` — all returned
correct results
4. **Spell-check compliance:** SKILL.md vocabulary reviewed against
repo's check-spelling dictionaries; replaced flagged words with standard
alternatives

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Co-authored-by: Copilot <[email protected]>
Co-authored-by: Copilot <[email protected]>