feat: default task automation to Luna

[liuxiaoyusky]

Summary

• default new project automations and legacy records without explicit model settings to gpt-5.6-luna with max reasoning
• make Luna use max reasoning when selected from the automation model catalog
• instruct automation runs to preserve explicit model or agent overrides and prefer luna_worker when it is available
• keep execution on the currently selected model when luna_worker is not installed, so the public workflow has no hidden custom-agent dependency

Existing records with explicit model and reasoning settings remain unchanged.

Validation

• npm test: 411 passed
• npm run typecheck
• npm run build:web