Config Phase 1 — shared `buildvana.json` configuration library + loader

[rdeago]

Sub-issue of #267.

Scope

• New host-agnostic library (Core-tier; System.Text.Json + BCL only, no MSBuild/git) holding the typed BuildvanaConfig model + section records and the loader. Referenced by Buildvana.Tool now and Buildvana.Sdk.Tasks later (Phase 6).
• Discovery anchored at the home directory: load buildvana.json xor buildvana.jsonc; fail if both are present. Absent file ⇒ all defaults.
• Parse options: ReadCommentHandling=Skip, AllowTrailingCommas, JsonUnmappedMemberHandling.Disallow (hard-fail unknown keys). A $schema member is whitelisted. Dictionary-typed sections (dotnet.args, nuget.feeds) get explicit key validation, since Disallow doesn't cover dictionary keys.
• Failures reported via BuildFailedException with clear messages.
• Register as a singleton in the tool's DI.
• Generate the JSON schema from the typed model (reusing [Description]s for hover docs), commit it, and add a test that regenerates and fails on staleness.
• SDK home-marker change: add buildvana.json/.jsonc as the top-priority marker (existence check only — the SDK does not parse it here), keep .buildvana-home and .git*, bail-if-both; apply identically in HomeDirectoryDiscovery and Sdk.props.

Acceptance criteria

• The library builds and is referenced by Buildvana.Tool; an absent config file yields defaults and no behavior change anywhere.
• A malformed file, an unknown key, an unknown dictionary key, or both .json+.jsonc present each fail with a clear message.
• The committed schema matches the model (staleness test passes); editors validate buildvana.json against it.
• Home-directory discovery recognizes buildvana.json/.jsonc and still recognizes .buildvana-home/.git*, in both the tool and the SDK.
• CHANGELOG: add an Unreleased entry introducing the (still inert) configuration file.