Config Phase 2 — wire existing `bv` options to `buildvana.json`

[rdeago]

Sub-issue of #267. Depends on Phase 1 (#268) and #280 (BuildPipeline extraction).

Scope

• Make the *Settings injectables config-aware: feed BuildvanaConfig into ReleaseSettings and the new per-step Settings (DotNetSettings + Restore/Build/Test/PackSettings, hanging off the BuildPipeline from Consolidate the build pipeline into a cohesive BuildPipeline #280), so each Resolve* establishes the flag → config → default chain. *Settings stay the source-merging injectables; config DTOs stay in Buildvana.Core.Configuration.
• Wire: dotnet.configuration and release.configuration (collapsing the two hardcoded "Release" defaults; resolution --configuration → release.configuration → dotnet.configuration → "Release", with release.configuration layering over DotNetSettings.Configuration); release.checkPublicApi; release.changelogUpdates (none|stable|all) + release.emptyChangelog (substitute text, else fail) — config-only, removing the --unstable-changelog and --require-changelog flags with no CLI replacement (changelog policy is repo-stable, not per-invocation); release.dogfood; dotnet.*.args and restore.*.env (where * is one of restore/build/test/pack/nugetPush, used according to the specific invocation in DotNetService) together with dotnet.all.args and dotnet.all.env (to be merged with invocation-specific settings by DotNetService.RunDotNetAsync()).
• release.generateDocsFrom (regex, short branch name) replaces mainBranch: delete GitService.MainBranch/FindMainBranch, the --main-branch global flag and its pre-parsing, and the discovery dance; the changelog permalink uses the actual CurrentBranch.

Acceptance criteria

• Each option resolves flag-over-config-over-default; with no config file, behavior matches today (except the removed --main-branch).
• changelogUpdates: none skips changelog updates; an empty Unreleased section is substituted from emptyChangelog when present, else fails the release.
• Docs generation is gated by generateDocsFrom (default ["^main$", "^master$"] when unconfigured, matching today's main/master discovery); --main-branch and MainBranch no longer exist.
• CHANGELOG: Unreleased entries for the new options; **BREAKING CHANGE** for the removed --main-branch flag.