feat: Add scenario/mod hook system (Issue #483)

[PipFoweraker]

Summary

Implements a minimal but complete scenario loading system that allows custom game configurations without code changes. This demonstrates expansion promise - content can scale beyond solo dev.

Key Features:

• ScenarioLoader class for loading scenario JSON files from godot/data/scenarios/ or user://scenarios/
• Scenario selection dropdown in Custom Game setup screen
• Support for overriding starting resources (money, compute, reputation, doom, etc.)
• Support for custom events in scenarios
• Support for custom start dates

Sample Scenarios Included:

• Bootstrap Mode: Extra resources for learning the game
• Crisis Mode: Challenging start with higher doom (2020 start date)
• Sandbox Mode: Unlimited resources for experimentation

Documentation:

• Complete format specification in docs/SCENARIOS.md
• Examples for common use cases
• Troubleshooting guide

Technical Changes

File	Change
godot/scripts/data/scenario_loader.gd	New ScenarioLoader class
godot/autoload/game_config.gd	Added scenario_id field with save/load
godot/scripts/game_manager.gd	Added _apply_scenario_overrides()
godot/scripts/core/events.gd	Extended to include scenario events
godot/scripts/ui/pregame_setup.gd	Scenario dropdown and description
godot/scenes/pregame_setup.tscn	UI elements for scenario selection
docs/SCENARIOS.md	Complete documentation
godot/data/scenarios/*.json	3 sample scenarios

Test Plan

• Launch game and navigate to Custom Game
• Verify "Scenario" dropdown appears with 4 options (Standard + 3 custom)
• Select each scenario and verify description updates
• Launch Bootstrap Mode - verify increased starting resources
• Launch Crisis Mode - verify reduced resources, higher doom, 2020 start date
• Launch Sandbox Mode - verify very high resources
• Verify scenario selection persists after game restart
• Drop a new .json file in scenarios folder and verify it appears without code changes

Closes #483

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