test: add coverage for autofarm, autogems, autoclothing, overlay

[Alistair-Afton]

Summary

• Add in-engine regression tests for autofarm (status, enable/disable via status, runonce, arg validation, bad plant id), autogems (config reload, enable/disable hook round-trip), autoclothing (status, arg validation), and overlay (list, widget enable/disable verified against get_state().config, unknown command)
• stockflow was considered but skipped: like mode, it is commented out of plugins/CMakeLists.txt and not part of the build
• overlay command output is suppressed under run_command_silent (quiet mode), so tests verify overlay_config state directly; autofarm enable/disable is silent by design, so tests assert via status

Test plan

• Full in-engine suite green: 300/301 tests passing with only a stale-deployed stocks.lua in my local game dir causing one failure; all new tests pass (Windows, DF 53.16)
• ci/lint.py clean on all new files