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.

.editorconfig

@@ -0,0 +1,114 @@
+# EditorConfig for DodoSSH — https://editorconfig.org
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+indent_style = space
+indent_size = 4
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[*.{json,yml,yaml,js,ts,css,html,axaml,xaml,csproj,props,targets,slnx}]
+indent_size = 2
+
+[*.md]
+# Two trailing spaces are a hard line break in Markdown.
+trim_trailing_whitespace = false
+
+[*.{cmd,bat,ps1}]
+end_of_line = crlf
+
+[*.cs]
+indent_size = 4
+
+#### Language conventions ####
+
+csharp_style_namespace_declarations = file_scoped:error
+csharp_using_directive_placement = outside_namespace:error
+csharp_style_var_for_built_in_types = false:suggestion
+csharp_style_var_when_type_is_apparent = true:suggestion
+csharp_style_var_elsewhere = false:suggestion
+csharp_prefer_braces = true:suggestion
+csharp_style_prefer_primary_constructors = true:suggestion
+csharp_style_expression_bodied_methods = when_on_single_line:suggestion
+csharp_style_expression_bodied_properties = true:suggestion
+
+dotnet_sort_system_directives_first = true
+dotnet_separate_import_directive_groups = false
+
+dotnet_style_qualification_for_field = false:suggestion
+dotnet_style_qualification_for_property = false:suggestion
+dotnet_style_qualification_for_method = false:suggestion
+dotnet_style_readonly_field = true:warning
+dotnet_style_require_accessibility_modifiers = for_non_interface_members:warning
+dotnet_style_coalesce_expression = true:suggestion
+dotnet_style_null_propagation = true:suggestion
+dotnet_style_prefer_is_null_check_over_reference_equality_method = true:suggestion
+
+# Async methods must be suffixed Async (VSTHRD200 equivalent via naming rules below).
+dotnet_naming_rule.async_methods_end_in_async.severity = warning
+dotnet_naming_rule.async_methods_end_in_async.symbols = any_async_method
+dotnet_naming_rule.async_methods_end_in_async.style = ends_with_async
+dotnet_naming_symbols.any_async_method.applicable_kinds = method
+dotnet_naming_symbols.any_async_method.required_modifiers = async
+dotnet_naming_style.ends_with_async.required_suffix = Async
+dotnet_naming_style.ends_with_async.capitalization = pascal_case
+
+dotnet_naming_rule.interfaces_start_with_i.severity = warning
+dotnet_naming_rule.interfaces_start_with_i.symbols = any_interface
+dotnet_naming_rule.interfaces_start_with_i.style = starts_with_i
+dotnet_naming_symbols.any_interface.applicable_kinds = interface
+dotnet_naming_style.starts_with_i.required_prefix = I
+dotnet_naming_style.starts_with_i.capitalization = pascal_case
+
+dotnet_naming_rule.private_fields_are_camel_case.severity = warning
+dotnet_naming_rule.private_fields_are_camel_case.symbols = private_field
+dotnet_naming_rule.private_fields_are_camel_case.style = camel_case_style
+dotnet_naming_symbols.private_field.applicable_kinds = field
+dotnet_naming_symbols.private_field.applicable_accessibilities = private
+dotnet_naming_style.camel_case_style.capitalization = camel_case
+
+#### Diagnostics ####
+
+# Formatting violations fail the build; `dotnet format --verify-no-changes` gates CI.
+dotnet_diagnostic.IDE0055.severity = error
+
+# ConfigureAwait is not meaningful in ASP.NET Core (no SynchronizationContext). It IS
+# meaningful in the Avalonia client, which re-enables CA2007 in its own .editorconfig.
+dotnet_diagnostic.CA2007.severity = none
+
+# Prefer LoggerMessage source generation over ILogger extension calls — allocation-free
+# and gives structured events by construction. Warning, so it is visible but not a wall
+# during early development; raised to error once the logging pass lands in M4.
+dotnet_diagnostic.CA1848.severity = warning
+
+# Exceptions carry ProblemDetails codes, not localised text.
+dotnet_diagnostic.CA1303.severity = none
+
+# CA1711 reserves the suffixes Flags, Permission, Collection, Stream and friends for
+# .NET Framework CAS and BCL base types that have no bearing on this codebase. The BCL
+# itself ships BindingFlags. PermissionFlags is the clearest name for a [Flags] enum of
+# permissions, and contorting domain vocabulary to satisfy a legacy rule costs more than
+# it returns.
+dotnet_diagnostic.CA1711.severity = none
+
+# CA1724 flags any type whose name collides with a BCL *namespace* (e.g. a type named
+# Permissions vs System.Security.Permissions). Namespace-qualified resolution makes this
+# a non-issue in practice and it heavily constrains domain naming.
+dotnet_diagnostic.CA1724.severity = none
+
+# We use file-scoped namespaces and modern C#; these fire on deliberate style choices.
+dotnet_diagnostic.CA1812.severity = none   # internal types instantiated by DI
+dotnet_diagnostic.CA1849.severity = warning # sync call in async method
+
+[tests/**/*.cs]
+# Test classes are instantiated by xunit, and test data is often public static.
+dotnet_diagnostic.CA1812.severity = none
+dotnet_diagnostic.CA1034.severity = none
+
+[src/DodoSSH.Infrastructure/Migrations/*.cs]
+# EF Core generates these; do not lint or format them.
+generated_code = true
+dotnet_analyzer_diagnostic.severity = none
+dotnet_diagnostic.IDE0055.severity = none