Add a way to sell the fishing boat

[dmccoystephenson]

Summary

Every other major asset in the game can be sold back for cash (home tiers via moveHome, investment properties via sellProperty), but the fishing boat was a one-way purchase with no resale path.

• Add resaleValue (70% of cost, matching the existing housing/investments ratio) to each BOAT_TIERS entry in src/business/business.py
• Add business.sellBoat(player), refunding the current tier's resale value and clearing hasBoat/boatTier/workers (any remaining crew is dismissed since they'd have nowhere to work)
• Wire a "Sell the Boat" action into Docks.manageBusiness() (src/location/docks.py), next to the existing "Dismiss a Worker" action, always available once a boat is owned
• Updated existing manageBusiness menu-index tests in tests/location/test_docks.py to account for the new menu option, and added dedicated tests for sellBoat (direct) and the menu action

Closes #108

Test plan

• python3 -m compileall -q src tests
• SDL_VIDEODRIVER=dummy SDL_AUDIODRIVER=dummy python3 -m pytest --verbose -vv --cov=src --cov-report=term-missing --cov-report=xml:cov.xml — 335 passed
• No front-end-specific code touched — this flows entirely through Docks.manageBusiness()'s shared BaseUserInterface.showOptions call, so console/pygame/web all get it automatically (same pattern the recent Retire-option PR used)

[dmccoystephenson]

Self-review: implementation matches the issue's suggested fix closely (70% resale value matching housing/investments, sellBoat() mirrors sellProperty()'s shape, dismisses remaining crew since they'd have nowhere to work). Caught and fixed two things during review: (1) inserting the new menu option unconditionally shifted every subsequent option index in Docks.manageBusiness(), which required updating 8 existing test side_effect sequences to keep them passing rather than just adding new tests; (2) a stray unrelated autoflake edit (dead-code removal in an unrelated test) got picked up while running the repo's formatter and was reverted before pushing, to keep the diff scoped to issue #108. Also updated the README's Fishing Business section, which described buying/hiring but not the new sell option, matching how Home Ownership/Investment Properties already document their resale paths. Full suite (335 tests) passes after each change. No front-end-specific code was touched, since this flows through the shared BaseUserInterface.showOptions abstraction Docks.manageBusiness() already uses, so console/pygame/web all pick it up automatically.