Doctor --pack flag silently produces zero checks for unregistered packs #177
Description
Context
mcs doctor --pack nonexistent silently displays "No packs detected (--pack flag)" with zero checks and a green summary. There is no warning that the named pack is not in the registry.
This applies to both the pre-existing behavior and the new --pack X --global combination from PR #175.
Proposal
After resolving the pack filter, check if the filtered pack IDs are a subset of registry.availablePacks identifiers. If any are missing, emit a warning like:
⚠ Pack "nonexistent" is not registered — no checks will be run for it
Impact
Low — not a crash, but a confusing silent no-op for users who mistype a pack name.