Move ~300 test functions from 21 catch-all files (handlers_core_test.go,
handlers_coverage*_test.go, *_coverage_test.go) into the *_test.go file
matching each handler's source file. This makes tests discoverable by
convention: tests for handlers_guild.go live in handlers_guild_test.go.
New files: handlers_guild_mission_test.go, sys_time_test.go.
No test logic changed — pure file reorganization.
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%.
