Restructures the agent-facing skills around the Copilot CLI / Claude
plugin layout (plugins/reactor/{plugin.json, agents/, skills/<name>/SKILL.md})
so the SDK's native skill-loading subsystem fires `skill.invoked` instead
of the agent treating SKILL.md as an ordinary file. Each topical skill
gets a focused SKILL.md with keyword-rich frontmatter; deeper content
lives under references/.

New skills (under plugins/reactor/skills/):
- reactor-getting-started — minimal single-file `#:package` starter, .csproj
  template, mode detection, package consumption, cache map fallback
- reactor-dsl — DSL essentials, hooks, components, theme tokens, gotchas;
  references/reactor.api.txt is the full alphabetized signatures index
- reactor-build-and-check — `mur check` usage, common-build-errors cheat
  table mapping REACTOR_* / CS* IDs to one-line fixes
- reactor-{async,design,forms,navigation,input,charts,commanding,devtools}
  — migrated from skills/<name>.md with plugin-format frontmatter
- reactor-recipes — paste-ready single-file recipes in references/

Local NuGet flow + agent kit:
- tools/Reactor.SignaturesGen — reflection-based generator emitting
  skills/reactor.api.txt from the built Reactor.dll. AfterBuild target
  skips when RuntimeIdentifier is set so cross-arch publishes don't fail.
- nuget.config + local-nupkgs/ — repo-level local feed
- mur pack-local — packs Microsoft.UI.Reactor.0.0.0-local.nupkg into the
  feed and clears NuGet's HTTP cache
- mur api / --regen-api — print / regenerate the signatures index
- mur check — runs `dotnet build` and emits one-line diagnostics with
  skill-file pointers for known REACTOR_* analyzer IDs
- Reactor.csproj packs the full plugin tree into agentkit/plugins/reactor/
  alongside the legacy SKILL.md + skills/ + reactor.api.txt

New analyzer:
- REACTOR_DSL_001 (MissingWithKeyAnalyzer) — flags Select(...) into a
  layout container without .WithKey on items

Recipes (skills/recipes/):
- async-fetch-list, form-with-validation, list-add-delete, sidebar-nav,
  themed-card — all compile against the local feed; verified end-to-end
  via c:\temp\nuget-test*.

Root SKILL.md is preserved as a fallback for environments without plugin
loading and now leads with a pointer to the plugin path.

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

CI was failing on `dotnet build Reactor.slnx` (windows-latest, x64 host)
because the slnx defaults Platform=ARM64 for projects that opt in. The
AfterBuild target then runs `Reactor.SignaturesGen.exe` — an ARM64 apphost
the x64 host cannot execute, exiting with code 216 ("not compatible with
the version of Windows you're running") and producing MSB3073.

The previous skip-condition only checked RuntimeIdentifier; this path
sets only Platform. Now resolve target arch from Platform/RID and host
arch from \$(NETCoreSdkRuntimeIdentifier), and skip regen when they differ.
The committed skills/reactor.api.txt is still picked up by pack.

Verified locally on win-arm64:
  -p:Platform=ARM64  → regen runs (target=arm64, host=arm64)
  -p:Platform=x64    → regen skipped (target=x64,  host=arm64)

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

`dotnet new reactorapp` requires both the framework nupkg and the
project-templates nupkg. Previously only the framework was packed; the
templates package was emitted only to tools/Templates/bin/Release/ as a
side effect of GeneratePackageOnBuild, leaving evals and dev workflows
to discover an undocumented path.

Now `mur pack-local` produces both packages in <repo>/local-nupkgs/ and
prints the dotnet-new install + create commands at the end. Includes a
note that consumers outside the clone need an absolute-path nuget.config
or a global feed registration, since the in-repo nuget.config uses a
relative path.

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