build(cli): skip SignaturesGen regen in CI to break the Reactor.Localization.Generator file-lock race

Root cause of the recurring CI "Unit Tests" failure on PR #437 and
sporadically on main: `src/Reactor.Cli/Reactor.Cli.csproj` declares a
`BeforeBuild` Exec target that shells out to a nested
`dotnet build Reactor.SignaturesGen.csproj`. SignaturesGen
ProjectReferences `Reactor.csproj`, which ProjectReferences
`Reactor.Localization.Generator`. The OUTER build (`dotnet test
tests/Reactor.Tests`) already produces
`obj/x64/Debug/netstandard2.0/Reactor.Localization.Generator.dll`
because `Reactor.Tests.csproj` directly references that project too.
The outer and nested `dotnet build` processes have their own
VBCSCompiler instances and don't coordinate — they race on the same
output dll. The second writer hits CSC error CS2012 ("Cannot open ...
for writing -- file may be locked by 'VBCSCompiler'"). The race window
scales with project size, which is why PR #437's increased compile
load triggers it consistently.

The committed `skills/reactor.api.txt` is what pack
(`Reactor.csproj:134`) and embed (`Reactor.Cli.csproj:84`) consume,
both gated `Condition="Exists(...)"`. CI never actually needs to regen
api.txt — it's a dev-machine convenience step.

Fix: add `and '$(CI)' != 'true'` to the RunSignaturesGen Target's
Condition. GitHub Actions sets `CI=true` as an env var; MSBuild picks
it up as `$(CI)`. Verified locally:
- `unset CI; dotnet build src/Reactor.Cli` — RunSignaturesGen fires
  (nested build runs, fast-up-to-date-checks skip everything since
  outputs are current).
- `CI=true dotnet build src/Reactor.Cli` — RunSignaturesGen fully
  skipped (zero "RunSignaturesGen" / "SignaturesGen.csproj" mentions
  in verbose log).

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

Author: codemonkeychris

src/Reactor.Cli/Reactor.Cli.csproj

@@ -109,9 +109,21 @@
        targets net10.0-windows10.0.22621.0. A ProjectReference across that TFM
        boundary fails NuGet asset resolution at pack time, so we shell out
        instead — SignaturesGen runs its own AfterBuild step that writes
-       skills/reactor.api.txt, which this csproj then embeds as a resource. -->
+       skills/reactor.api.txt, which this csproj then embeds as a resource.
+
+       Skip in CI ('$(CI)' = 'true' is set by GitHub Actions). The nested
+       `dotnet build` spawns its own VBCSCompiler instance that doesn't
+       coordinate with the outer build — when an outer test/build target
+       (e.g. `dotnet test tests/Reactor.Tests`, which itself ProjectReferences
+       Reactor.Localization.Generator) is already producing
+       obj/<plat>/<cfg>/<tfm>/Reactor.Localization.Generator.dll, the nested
+       SignaturesGen build races on the same output file and the second
+       writer loses with CSC error CS2012. The committed
+       skills/reactor.api.txt is what pack + embed consume (both via
+       Condition="Exists(...)"), so the regen is a dev-machine convenience —
+       CI never needs to run it. -->
   <Target Name="RunSignaturesGen" BeforeTargets="BeforeBuild"
-          Condition="'$(SkipSignaturesGen)' != 'true'">
+          Condition="'$(SkipSignaturesGen)' != 'true' and '$(CI)' != 'true'">
     <Exec Command="dotnet build &quot;$(MSBuildThisFileDirectory)..\..\tools\Reactor.SignaturesGen\Reactor.SignaturesGen.csproj&quot; -c $(Configuration) -p:Platform=$(_SignaturesGenPlatform) --nologo -v:q"
           ConsoleToMSBuild="true"
           ContinueOnError="false" />