[Future] diagnostics: revisit a safer oc-plugins doctor experience

The original PRD included an `oc-plugins doctor` command for local diagnostics, but active v1 scope no longer includes it.

This issue keeps that idea visible without letting it distort the active v1 milestone chain.

## Why deferred
- the original `doctor` concept drifted toward diagnostic certainty that is hard to justify statically
- the active v1 roadmap is centered on the core read path, registry enrichment, safe mutations, and polish
- diagnostics should return only if the scope and wording can stay narrow and trustworthy

## Not part of the active v1 milestone chain
- not required for M2 read path
- not required for M3 registry enrichment
- not required for M4 safe mutations
- not required for M5 polish

## Candidate future scope
- validate configured npm specs
- surface manifest or install-shape problems
- inspect declared OpenCode support metadata when present
- inspect expected server or TUI entry targets when the signal is trustworthy
- avoid hard compatibility claims or overstated health guarantees

## Product intent
The deferred diagnostics experience may come back as `doctor` or as a narrower health-check workflow, but it should be designed as future work rather than treated as unfinished v1 scope.