feat(template): support agk template remove by name

[kunalkushwaha]

What

agk template remove previously only accepted the full source path; passing a manifest name like rag-agent failed. This was a documented TODO in cmd/template.go. Now you can remove a cached template by its name.

This is feature A3 from the FEATURES.md roadmap.

Changes

• CacheManager.FindByName(ref) / RemoveByName(ref) — match cached templates by manifest name or source, removing all cached versions and reporting how many were removed.
• template remove <ref> tries the name/source lookup first, then falls back to source-path removal, with a clear template "<ref>" not found in cache error.

Before / After

$ agk template remove rag-agent
# before: error (only the full source path worked)
# after:  Removed template: rag-agent (1 cached version(s))

Testing

• go build, go vet, go test ./..., gofmt all green.
• Adds the registry package's first unit tests (FindByName by name and by source, multi-version RemoveByName, not-found).
• Verified end-to-end with an isolated HOME: list → remove by name → empty, plus the not-found error path.

Independent branch off main; does not touch README (so it won't conflict with the other open PRs).

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