Link to namespace extensions listing from profile pages

Problem Statement

After #446 shipped /extensions/@, there is no in-app path to discover that URL. Users must either type it by hand or be linked externally. Profile pages — /u/ for operatives and /o/ for collectives — are the natural entry points but have no link to the namespace listing.

Proposed Solution

Add an "Extensions" row (or similar single-line panel) to both /u/ and /o/ that:

• Displays the number of extensions published under the namespace (e.g. "Extensions: 7").
• Links the count / label to /extensions/@.
• Renders the empty case (0 extensions) either as plain text with no link, or omits the row entirely — both are acceptable; pick whichever is less visually noisy.

Scope

• New read method on ExtensionRepository: countByNamespace(namespace: string): Promise<number>.
• MongoDB implementation mirroring the existing namespace-prefix regex used in search().
• Wire through lib/app/repos.ts.
• Render the row on both profile pages.

Alternatives Considered

• Linking without a count — simpler but less informative; the count provides a useful "what am I clicking into" signal.
• Only linking from collective pages (not operative pages) — rejected because the /extensions/@ route resolves both kinds, and operatives can own namespaces too.

Context

This was deferred from PR #446 per adversarial review (ADV-2) to keep the initial URL work scoped.