feat: Cache paginated GitHub tags per repository

[nikolaik]

Aggregate and cache all tag pages per owner/repo in github.Service so
multiple modules sharing a repo (and repeated calls within the TTL) reuse
a single paginated fetch instead of re-walking every page.

[Copilot]

Pull request overview

This PR adds per-repository caching of GitHub tag pagination results in github.Service so multiple module lookups against the same owner/repo can reuse one aggregated tags fetch within a TTL, instead of re-walking every page each time.

Changes:

• Introduces a generic mcache.Noop cache implementation to allow disabling caching while still satisfying a Get/Set-style interface.
• Refactors github.Service.ListVersions to fetch all tag names via a new listTags helper and caches those tag names per owner/repo.
• Updates server wiring and expands GitHub service tests to cover caching and pagination behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
pkg/mcache/mcache.go	Adds a no-op cache implementation to make caching optional via interface injection.
pkg/github/github.go	Adds TagCache, injects it into Service, and caches aggregated tag names per repository.
pkg/github/github_test.go	Updates constructor usage and adds tests for per-repo caching and multi-page pagination.
cmd/server/main.go	Wires an in-memory tag cache (or a no-op cache when disabled) into the GitHub service.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.