Account hiding flag, subagent timeouts, and TUI fixes

Added AGY_CLI_HIDE_ACCOUNT_INFO flag, subagent-specific interaction timeouts, and TUI fixes.

• Added AGY_CLI_HIDE_ACCOUNT_INFO environment variable to hide email and plan tier from the header.
• Improved /help shortcuts tab by sorting shortcuts by keybinding key, adding missing keybindings (like ctrl+r, ctrl+o, alt+j, ctrl+k), and generalizing scrolling (PageUp/PageDown/GoToTop/GoToBottom) for both Commands and Shortcuts tabs.
• Fixed timeout overrides: restricted the default 60-second interaction timeout specifically to subagents, preventing the main agent from being unconditionally capped.
• Fixed a nil-pointer panic in Sandbox Mode: resolved a typed nil interface comparison when fetching URL content.
• Fixed fallback skill discovery in Standalone mode: ensures custom/fallback skills are successfully loaded even if the standard configuration directory is missing, and added automatic path deduplication to prevent duplicates.
• Fixed command rendering in message history: prefixed slash commands with a caret (>) in response block headers to clearly distinguish user-typed commands from agent outputs.
• Fixed plugin installation path mismatch: updated the plugin subcommand to install downloaded plugins directly to the shared configuration directory (~/.gemini/config/) rather than the private application data folder, making them instantly discoverable.
• Fixed Git short-hash support in diff selection: updated the commit hash recognition pattern in the /diff commit selection tree to match Git's standard 7-character short hashes (and up to 40-character full hashes).
• Fixed statusline subcommand handling and recursive loops: added case-insensitive subcommand parsing (help, delete, reset, enable/on, disable/off) to the /statusline command, providing direct control to toggle or revert custom statuslines and blocking recursive shell hangs during help queries.