Workspace-grouped resume view, Gemini 3.8 Flash support, and Markdown agent ambient customization inheritance

Adds an opt-in workspace-grouped view to the /resume picker with Ctrl+F toggling, adds Gemini 3.8 Flash to the model catalog for GEMINI_API_KEY users, updates Markdown-defined custom agents to inherit ambient skills, rules, and subagents by default, and delivers fixes for long MCP OAuth authorization codes, Windows skill path separators, /boost worker tools, and runtime nil pointer crashes.

• Added an opt-in workspace-grouped view to the /resume conversation picker, allowing users to toggle between a flat list and conversations grouped by directory with Ctrl+F.
• Added Gemini 3.8 Flash to the model catalog when connecting with a GEMINI_API_KEY.
• Changed custom agents defined in Markdown to inherit ambient skills, rules, and subagents by default, matching the configuration of default agents.
• Fixed MCP OAuth authentication failing when authorization servers return authorization codes longer than 1024 characters.
• Fixed skill path matching and grouping in the /skills panel on Windows where host path separators (\) caused global and workspace skills to be misclassified.
• Fixed relative path resolution in Markdown-defined custom agents so relative subagent and plugin paths resolve correctly against the agent definition's directory.
• Fixed duplicate permission grants accumulating in configuration settings across session reloads and subagent invocations.
• Fixed /boost failing at runtime by improving worker tool configurations.
• Fixed a fatal nil pointer crash in the agent runtime caused by background summary updates attempting to read cached steps after trajectory closure.
• Hardened Remote Control reverse-tunnel routing.