sharing 7/7: Game modes export (widget, headless startscript, CI workflow) - #8095

📚 The sharing stack — review bottom-up

• 1/7 · Transfer library (enums, comms, serialization, test infra) (sharing 1/7: Transfer library (enums, comms, serialization, test infra) #8125)
• 2/7 · Modes & economy boundary (presets, waterfill, share ledger) (sharing 2/7: Modes & economy boundary (presets, waterfill, share ledger) #8126)
• 3/7 · Policies (context -> policy -> policy_result -> action) (sharing 3/7: Policies (context → policy → policy_result → action) #8407)
• 4/7 · Transfer runtime (controllers, economy boundary, gadget swap) (sharing 4/7: Transfer runtime (controllers, economy boundary, gadget swap) #8062)
• 5/7 · Tech Core / Keystone (sharing 5/7: Tech Core / Keystone #8063)
• 6/7 · Sharing Tab UI (sharing 6/7: Sharing Tab UI #8064)
• 7/7 · Game modes export (export widget, headless startscript, CI workflow) (sharing 7/7: Game modes export (widget, headless startscript, CI workflow) #8095) ← you are here

Each PR is file-partitioned: every file appears in exactly one PR in its final sharing_tab form, so each PR's diff is byte-identical to that branch. Regenerated deterministically by just bar::sharing-split.

Summary (LLM-generated, claude-opus-4-6)

Adds the export pipeline that makes game mode presets available to out-of-engine consumers (the SPADS ModeCommand plugin). A LuaUI widget (export_game_modes.lua) collects modoption defaults by section, overlays each modes/<category>/*.lua preset to build effective modOptions (with locked flags), and writes the result as structured JSON (game_modes.json, schemaVersion 1). A headless startscript enables the widget via debugcommands, runs /exportgamemodes, and quits. The CI workflow (export_game_modes.yml) runs on push to stable/master and workflow_dispatch, resolving the engine from launcher config, running spring-headless --isolation, validating the output, and publishing channel-rolling (game_modes-stable.json / game_modes-test.json) or commit-pinned assets to a game-modes GitHub release.