[SDK-11] Migrate first-party CI host adapters onto Fallout.Plugin.Sdk as dogfood proof

The [roadmap](https://github.com/ChrisonSimtian/Fallout/blob/main/docs/roadmap.md#v12--public-plugin-sdk) commits to "First-party CI host adapters migrate to the SDK as dogfood proof" but no SDK-* issue tracks this — it's distinct from #158 (single canonical reference plugin).

## Scope

Migrate the ~12 first-party CI host integrations currently in `Fallout.Common.CI.*` (GitHub Actions, Azure Pipelines, TeamCity, Jenkins, Bitrise, AppVeyor, Travis, GitLab, Bamboo, Space, Bitbucket, …) onto the public `Fallout.Plugin.Sdk` extension points.

- Each adapter becomes a separately versioned NuGet package (e.g. `Fallout.Plugin.CI.GitHub`).
- Migration uses `IHost` / `IHostDetector` (#152) and `ITargetLifecycleListener` (#155).
- Existing consumer-facing types stay source-compatible — the migration is internal.

## Why this matters

- Proves the SDK is fit for purpose against the most complex first-party use case before declaring 1.0.
- Forces every SDK-6 extension point to face a real-world integration, not just a contrived example.
- Lets us delete a chunk of `Fallout.Common.CI.*` once the plugin packages ship.

## Depends on

- All of SDK-6a..6f (#151–#156)
- SDK-1 (#146), SDK-2 (#147), SDK-3 (#148)

## Done when

- [ ] At least 3 first-party CI hosts ship as `Fallout.Plugin.CI.*` NuGet packages
- [ ] CI adapter tests run against the plugin-loaded versions
- [ ] Build output unchanged for consumers using the new plugin packages