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.

src/DodoSSH.Contracts/PublicAPI.Unshipped.txt

@@ -0,0 +1,14 @@
+#nullable enable
+DodoSSH.Contracts.ProblemCodes
+const DodoSSH.Contracts.ProblemCodes.AlreadyEnrolled = "already-enrolled" -> string!
+const DodoSSH.Contracts.ProblemCodes.ClientTooOld = "client-too-old" -> string!
+const DodoSSH.Contracts.ProblemCodes.EnrollmentRequired = "enrollment-required" -> string!
+const DodoSSH.Contracts.ProblemCodes.Forbidden = "forbidden" -> string!
+const DodoSSH.Contracts.ProblemCodes.IdempotencyKeyReuse = "idempotency-key-reuse" -> string!
+const DodoSSH.Contracts.ProblemCodes.InvalidCursor = "invalid-cursor" -> string!
+const DodoSSH.Contracts.ProblemCodes.PushBatchTooLarge = "push-batch-too-large" -> string!
+const DodoSSH.Contracts.ProblemCodes.RelayLimitReached = "relay-limit-reached" -> string!
+const DodoSSH.Contracts.ProblemCodes.RelayTargetRejected = "relay-target-rejected" -> string!
+const DodoSSH.Contracts.ProblemCodes.RelayTicketInvalid = "relay-ticket-invalid" -> string!
+const DodoSSH.Contracts.ProblemCodes.TypeBaseUri = "https://dodossh.dev/problems/" -> string!
+const DodoSSH.Contracts.ProblemCodes.VaultConflict = "vault-conflict" -> string!