Create the v2 solution skeleton

[pournasserian]

Closes #2433

What

The solution and the projects from ADR-0004, as empty shells with the build rules enforced from day one.

• FluentCMS.slnx with src/ and tests/ folders, Directory.Build.props (net10.0, nullable, implicit usings, warnings as errors, deterministic), Directory.Packages.props (central package versions), nuget.config (nuget.org only).
• FluentCMS.Core, FluentCMS.Infrastructure, FluentCMS.Mcp, FluentCMS.Api, FluentCMS.Admin and FluentCMS.Host, each with only the references its ADR-0004 row permits and a composition seam (AddFluentCms*) for the epics to fill.
• Host entry modes: fluentcms is the web host with JSON console logging and a /health endpoint, nothing else; fluentcms mcp is a generic host with the same composition, logs to standard error, binds no port, and stops when standard input closes.
• Infrastructure: an empty FluentCmsDbContext, provider selection by configuration (SQLite by default, Npgsql when the connection string says Host= or Database:Provider is postgresql), and a startup migrator that is a no-op until the first migration exists.
• Job queue stub: a hosted worker pool from ADR-0010 that idles on the polling interval and stops cleanly.
• Tests: a reference-rule test that reads every src/*.csproj and fails on a direction ADR-0004 forbids; integration tests for the web host (health endpoint, job queue registered), the stdio host (no IServer, exits when input closes), SQLite, an unknown provider, and PostgreSQL when FLUENTCMS_TEST_POSTGRES is set (skipped otherwise).
• CI: a build-and-test job beside the markdown lint, with a PostgreSQL 17 service container and the NuGet cache keyed on Directory.Packages.props.
• CONTRIBUTING.md now documents build, test and run commands; .gitignore covers local SQLite files.

Why

Every M1 epic needs a place to put code and a build that keeps the dependency direction honest (#2433, ADR-0004). The two host modes and the database shell let the first epics start on the same composition the product will ship with.

Verified locally

dotnet build with warnings as errors, dotnet test (PostgreSQL test skipped), both host modes as real processes, and the rule test failing when a forbidden reference is added to the Admin project.

Checklist

• Linked to an issue a maintainer has confirmed
• Targets v2
• Tests or docs updated where behaviour changed
• No tool-generated co-author trailers or footers