API: one dashboard read to replace the per-repo fan-out #41

The iOS app's dashboard ("what needs me") needs, per account:

• open merge requests on repos I can reach
• open issues assigned to me
• recent builds
• pinned repos — repo pin/repo unpin exist, but no read command exposes the pin list

Measured on gitbay.org with 66 repos: repo list + mr list + issue list per repo, plus build list where something is open, is ~130 reads. The 120/min bucket ran dry at repo 64 on the first load. ETag revalidation cuts bytes, not budget.

Proposal: a read-only dashboard control command returning {pinned repos, open MRs, assigned issues, recent builds} for the calling account — one call. The web dashboard already computes this server-side (store.PinnedRepos and friends); exposing it through the registry gives the CLI the same aggregate and keeps the surfaces from drifting.

Ref #11