Adds Repo.SetUserCaps(login, caps) and a Hub delegate, wrapping libfossil's Repo.SetCaps. Lets consumers grant or change caps on existing users without leaving the hub package — primarily for the common bones case where the libfossil-pre-populated 'nobody' user needs 'gio' to allow unauthenticated clone/pull/push.

Also adds Config.NobodyCaps so the typical setup is one declarative field instead of a manual SetUserCaps call after every NewHub. NobodyCaps is applied at bootstrap by calling SetUserCaps("nobody", v) internally — no method-body duplication. Empty NobodyCaps leaves nobody at whatever libfossil set it to (idempotent re-open path preserves prior caps, doesn't overwrite).

Tests: - TestRepo_SetUserCaps_RoundTrip — set + read back - TestRepo_SetUserCaps_RejectsEmptyLogin - TestNewHub_NobodyCaps_AppliedAtBootstrap — Config field works - TestNewHub_NobodyCaps_Idempotent — re-running with same caps OK - TestNewHub_NobodyCaps_EmptyDoesNotOverwrite — empty doesn't reset

Closes #130 leaf check-in: a8e62ce82d user: [email protected] tags: trunk, feat/130-hub-setusercaps