feat: add SKILL_ENV support and update skill settings

[joyyile511]

Summary

• Add {SKILL_ENV} placeholder support in skills.service.ts so skills can reference their own .env file path without hardcoding
• Expose skill env API through preload layer (index.ts / index.d.ts)
• Update SkillsSettings.tsx to support skill-level env configuration in the Settings UI
• Increase bash tool default timeout from 300s to 600s to prevent premature termination of long-running skill scripts

Changes

File	Change
skills.service.ts	Replace {SKILL_ENV} placeholder with skill's .env file path
skills.handlers.ts	IPC handler updates for skill env support
common.ts	Bash tool timeout: 300000ms → 600000ms
preload/index.ts	Expose skill env API to renderer
preload/index.d.ts	Type definitions for skill env API
SkillsSettings.tsx	UI support for skill env configuration

Test plan

• Verify {SKILL_ENV} is correctly resolved in skill SKILL.md at runtime
• Confirm skill .env file path is accessible from renderer via preload
• Test long-running skill scripts complete without timeout errors
• Verify Settings → Skills shows env configuration correctly