Plugin CLI inspector — `fallout plugins list/info` commands

Once plugins are a real thing, users will want a way to inspect what's loaded into their build. Filing as backlog/discretionary — not v12-blocking, but a near-certain follow-up.

## Sketch

- `fallout plugins list` — show installed plugin packages with version, source (NuGet vs. local), and whether they loaded successfully.
- `fallout plugins info ` — show the contributions each plugin makes (middlewares registered, CI hosts contributed, parameter sources, etc.).
- `fallout plugins doctor` (stretch) — diagnose load failures, version mismatches, and conflict-resolution outcomes.

## Why backlog, not v12

- Plugins authoring + loading is the v12 critical path; introspection can ship 12.1.
- Until plugin volume grows, the diagnostic value is small.
- Risk: if discovery/load (#100, #148) shapes don't expose enough metadata, retrofitting `info` requires expanding the SDK surface — keep this issue open during v12 implementation as a forcing function for the metadata model.

## Related

- v12 SDK milestone (#7)
- Discovery & load (#100, #148)
- Plugin telemetry & diagnostics framework (#161) — different layer (runtime tracing, not author-facing CLI), but adjacent