docs: point nine stale paths at the code that exists

[melbinjp]

Closes the rest of #6387, which is assigned to me. #6388 took the five plain renames; these are the nine that needed a decision about what the replacement should say.

Re-measured against e532cf0 rather than re-quoted from the issue. Two things changed since I filed it: one line had moved again, and pkg/skills/feature_gate.go is already gone from the docs, so it is nine rather than the ten I last counted.

Four moved to toolhive-core

The replacement is the import path the Go code uses today, not a guess.

document	was	now
docs/arch/03-transport-architecture.md	pkg/permissions/profile.go	toolhive-core/permissions
docs/arch/06-registry-system.md	pkg/registry/types.go	toolhive-core/registry/types
docs/arch/06-registry-system.md	pkg/container/verifier/	toolhive-core/container/verifier
docs/error-handling.md	pkg/errors/errors.go	toolhive-core/httperr

Two of these are self-evidencing. pkg/runner/retriever/retriever.go imports toolhive-core/container/verifier, and docs/error-handling.md already uses httperr.Code(err) in the example directly above the list that still named pkg/errors.

pkg/registry/schema_validation.go is named as the validator for schemas the same document already says are published in toolhive-core, so it points there too.

Discovery

pkg/vmcp/discovery/ is not in the tree. Discovery is, in pkg/vmcp/aggregator/discoverer.go, so the three references name the aggregator rather than dropping the idea. The vmcp-library.md row is removed: it lists a package that no longer exists, and it already told readers to use it via the aggregator.

One left alone deliberately

docs/arch/10-virtual-mcp-architecture.md still has a Backend Enrichment Middleware section naming pkg/vmcp/server/backend_enrichment.go. That file went in #5622 and nothing in the tree replaces it, so unlike the others this is not a path that moved, it is a section describing something that is gone. Deleting a section is an editorial call rather than a path fix, so I have left it for you. Happy to send it as a one-line follow-up if you would like it removed.

Verification

All four toolhive-core links resolve, and all five in-tree paths are present at e532cf0. Docs only, no code changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.91%. Comparing base (e532cf0) to head (6b203ba).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6650      +/-   ##
==========================================
- Coverage   78.98%   78.91%   -0.07%     
==========================================
  Files         782      782              
  Lines       78065    78065              
==========================================
- Hits        61658    61607      -51     
- Misses      16402    16453      +51     
  Partials        5        5              

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