feat(sitecore-context): add ContentDatabase, Site, Language, User to Axlis.Sitecore.Context

[marioarce]

Summary

Extends Axlis.Sitecore.Context (published v0.1.0) with four new properties, captured per-request using the same thread-safe mechanism already used for .Database, .Request, and .HttpContext:

• ContentDatabase — Sitecore.Data.Database
• Site — Sitecore.Sites.SiteContext
• Language — Sitecore.Globalization.Language
• User — Sitecore.Security.Accounts.User

Design follows the signed-off Axlis - SitecoreContext - Implementation Plan 2 (Final).md: all seven members (including the original three) are captured once in SitecoreContextHttpModule.OnBeginRequest and served from the thread-safe store for the rest of the request. Unlike Database/Request/HttpContext, Site/Language/User can legitimately change mid-request (language switch, login/impersonation, site switch) — this is a deliberate snapshot trade-off, not an oversight, and is documented in XML docs, both package READMEs, and docs/sitecore-context/Architecture.md.

This PR ships the package-level change only. samples/Axlis.Sitecore.Context.Samples.sln updates are a deferred follow-up. Version bump to 1.0.0 and the NuGet/GitHub Release happen later, in the release branch per SOP 5 in WORKFLOWS.md — not part of this PR.

Changes

• Context.cs: 4 new public static properties, CapturedContext grown to 7 fields, Capture(...) grown to 7 params, new fallback helpers, extensive XML doc <remarks> on the snapshot semantics.
• SitecoreContextHttpModule.cs: OnBeginRequest now captures and stores all 7 values.
• Axlis.Sitecore.Context.Sitecore102/README.md: documents the 4 new properties and the snapshot-semantics trade-off; extends the manual verification checklist.
• docs/sitecore-context/Architecture.md: new section explaining why Site/Language/User are captured the same way despite being commonly mutated mid-request.
• Root README.md: package-family table updated to list all 7 members.
• Axlis.Sitecore.Context.Sitecore102.csproj: <Description> updated.
• AmbientContextStoreTests.cs (Axlis.Sitecore.Context.Abstractions.Tests): fixed 10 pre-existing warnings surfaced during local build verification (missing XML doc comments, a possible-null-reference return, and 3 unnecessary ConfigureAwait(false) calls in test methods) — required for the CONTRIBUTING.md Cross-Cutting Gate (0 warnings/0 errors). Out of this PR's original scope but fixed per explicit request.
• .github/workflows/ci.yml: added build-customizations and build-sitecore-context jobs. CLAUDE.md/CHANGELOG.md already documented these jobs as existing, but only the original Axlis.ORM build job had ever been added — this closes that documentation/reality gap, and the job names now match what WORKFLOWS.md SOP 5 references.

Local build (Visual Studio): 3 succeeded, 0 failed, 0 errors, 0 warnings.

Test plan

• Local build succeeds with 0 warnings, 0 errors (confirmed by Mario)
• CI passes on this PR, including the new build-sitecore-context job
• Manual verification against a real Sitecore 10.2.x instance (per README checklist) — to be run by Mario before the release branch is cut

Closes #53