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

[capyBearista]

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.