Move business logic for guild disband, resign leadership, leave,
post scout, and answer scout from handlers into GuildService methods.
Handlers now delegate to the service layer and handle only protocol
concerns (packet parsing, ACK responses, cross-channel notifications).
Adds 22 new table-driven service tests and sentinel errors for typed
error handling (ErrNoEligibleLeader, ErrAlreadyInvited, etc.).
DonateRP left in handler due to Session coupling.
Cover the 4 handler files that had no tests: handlers_guild_ops.go,
handlers_guild_scout.go, handlers_guild_board.go, and handlers_items.go.
44 new tests exercise the error-logging paths added in 8fe6f60 and the
core handler logic (disband, resign, apply, leave, accept/reject/kick,
scout answer, message board CRUD, weekly stamps, item box parsing).
New mock types: mockGuildRepoOps (enhanced guild repo with configurable
errors and state tracking), mockUserRepoForItems, mockStampRepoForItems,
mockHouseRepoForItems. Coverage rises from 41.1% to 43.7%.
