MCP panel navigation improvements, multi-line comment support in mcp_config.json, and headless stream cleanup

Improves /mcp panel arrow key navigation and action cycling, adds support for comments and trailing commas in mcp_config.json, ensures headless CLI streams close cleanly on exit, and delivers fixes for duplicate agent listings, deleted working directory startup, and /btw side questions with active goals.

• Improved /mcp panel navigation so up and down arrow keys strictly navigate between MCP servers and plugins, left and right arrow keys cycle available actions, and Esc exits action mode or the panel.
• Fixed duplicate agent entries appearing in the /agents picker when an agent with the same name is discovered across multiple configuration sources or plugins.
• Fixed tool initialization and agent startup failing when launched from an inaccessible or deleted working directory by using absolute URI schemes for in-memory parameter schemas.
• Fixed headless CLI invocations with piped standard output or standard error hanging on exit by setting FD_CLOEXEC on the preserved streams so child processes do not keep the caller's pipes open.
• Fixed MCP configuration parsing failing when mcp_config.json contains single-line (//) comments, multi-line (/ /) comments, or trailing commas.
• Fixed orphaned annotation files accumulating in local storage when deleting conversations.
• Fixed /btw side questions in conversations with an active /goal being forced to continue and making unintended tool calls.