Restructure into src/tests and add build foundation (M0)

Moves the scaffold to src/DodoSSH.Api and establishes the repo conventions the rest
of the milestones build on.

Structure:
- src/{Contracts,Crypto,Domain,Infrastructure,Api}, tests/{Contracts,Crypto,Domain}.Tests
- DodoSSH.slnx rewritten with src/ and tests/ solution folders

Build:
- Directory.Build.props centralises TFM, nullable, deterministic builds and
  TreatWarningsAsErrors; Directory.Packages.props pins every version centrally
- packages.lock.json committed so CI restores in locked mode
- NuGet.config clears machine-level sources, which both fixes NU1507 under central
  package management and makes restore reproducible off this machine
- Microsoft.OpenApi pinned to 2.11.0: ASP.NET Core 10.0.10 resolves 2.0.0, which is
  covered by GHSA-v5pm-xwqc-g5wc (high, patched in 2.7.5)

Analyzers:
- AnalysisLevel is Recommended, not All. With warnings-as-errors, All turns opinionated
  naming rules into build breaks and trains people to blanket-suppress.
- BannedSymbols.txt bans DateTime.UtcNow (TimeProvider), Guid.NewGuid (CreateVersion7),
  sync-over-async, MD5/SHA1, PBKDF2 and SecureString
- CA1711/CA1724 disabled: both are .NET Framework CAS-era naming rules
- PublicApiAnalyzers on Contracts only, since that assembly is the client's real contract

API:
- weather-forecast template removed
- UseHttpsRedirection removed; TLS terminates at the reverse proxy and redirecting
  behind one causes loops
- /healthz/{live,ready,startup}. Liveness deliberately checks no dependencies so a
  transient database outage cannot restart the container and kill live SSH sessions.

Notes:
- No coverage collector yet. Microsoft.Testing.Extensions.CodeCoverage pulls an MTP 1.x
  MSBuild extension that throws TypeLoadException against the MTP 2.3.x xunit.v3 brings.
  Coverage gates are an M3 concern; revisit with an MTP 2.x-aligned version then.

Verified: dotnet build (0 warnings), 17 tests pass, format check clean, API serves
health and OpenAPI endpoints.

Directory.Packages.props

@@ -0,0 +1,52 @@
+<Project>
+
+  <PropertyGroup>
+    <ManagePackageVersionsCentrally>true</ManagePackageVersionsCentrally>
+    <CentralPackageTransitivePinningEnabled>true</CentralPackageTransitivePinningEnabled>
+  </PropertyGroup>
+
+  <!--
+    Versions are pinned here for the whole solution. Packages are added per milestone
+    rather than all at once, so that every entry is one we have actually verified and
+    restored. See docs/adr/ for the choices behind the notable ones.
+  -->
+
+  <ItemGroup Label="ASP.NET Core">
+    <PackageVersion Include="Microsoft.AspNetCore.OpenApi" Version="10.0.10" />
+  </ItemGroup>
+
+  <ItemGroup Label="Pinned transitive dependencies">
+    <!--
+      Microsoft.AspNetCore.OpenApi 10.0.10 resolves Microsoft.OpenApi 2.0.0, which is
+      covered by GHSA-v5pm-xwqc-g5wc (high: circular schema references can terminate
+      OpenAPI parsing; vulnerable <= 2.7.4, patched in 2.7.5). Pinned forward within the
+      2.x major that ASP.NET Core 10 targets. Revisit when the ASP.NET Core package
+      itself moves off 2.0.0.
+    -->
+    <PackageVersion Include="Microsoft.OpenApi" Version="2.11.0" />
+  </ItemGroup>
+
+  <ItemGroup Label="Analyzers">
+    <PackageVersion Include="Microsoft.CodeAnalysis.BannedApiAnalyzers" Version="5.6.0" />
+    <PackageVersion Include="Microsoft.CodeAnalysis.PublicApiAnalyzers" Version="5.6.0" />
+    <PackageVersion Include="Meziantou.Analyzer" Version="3.0.134" />
+  </ItemGroup>
+
+  <ItemGroup Label="Testing">
+    <!--
+      xunit.v3 runs on Microsoft.Testing.Platform, not VSTest. Microsoft.NET.Test.Sdk and
+      coverlet.collector are VSTest components: referencing them alongside MTP raises
+      MTP0001 and their collector never runs, so neither is referenced.
+
+      No coverage collector yet. Microsoft.Testing.Extensions.CodeCoverage 18.9.0 pulls
+      Microsoft.Testing.Platform.MSBuild 1.9.1, which is built against MTP 1.x and throws
+      TypeLoadException on IDataConsumer against the MTP 2.3.x that xunit.v3 3.2.2 brings.
+      Coverage gates are an M3 concern (90% on Domain and Authorization); pick a version
+      aligned with MTP 2.x then rather than carrying a broken dependency until it matters.
+    -->
+    <PackageVersion Include="xunit.v3" Version="3.2.2" />
+    <PackageVersion Include="Shouldly" Version="4.3.0" />
+    <PackageVersion Include="NSubstitute" Version="6.0.0" />
+  </ItemGroup>
+
+</Project>