Reasoning effort command, model slug pinning, subagent model configuration, and TUI stability improvements

Added /effort command and --effort flag for reasoning effort selection, stable model slugs, subagent model config, redesigned /model picker, scrollable /settings panel, and various MCP and background task fixes.

• Added a /effort command to view and change the current model's reasoning effort, with a left/right timeline-gauge picker and a direct /effort <level> form so you can trade latency for depth on the fly.
• Added an --effort flag to select a model's reasoning-effort variant when launching the CLI.
• Added stable, user-facing model slugs that appear in the /model picker and are accepted by --model, so you can pin a specific model reliably across sessions.
• Added a model option to custom agent frontmatter so an agent runs at a chosen model tier (such as flash or pro) when invoked as a subagent, defaulting to inherit (the parent's model).
• Redesigned the /model picker to group models by their base model and choose reasoning effort from a timeline gauge navigable with Left and Right, and added an effort badge to the status line for models that expose multiple effort variants.
• Improved the /settings (/config) panel by making it a bounded, scrollable list so it renders correctly in short terminals instead of overflowing, and stopped it from flickering when opening and closing dropdowns.
• Improved background-task reliability by moving long-running work onto a shared lifecycle with deterministic startup and shutdown and panic-safe launching, so a failure in one background task no longer disrupts the session and pending analytics are flushed on exit instead of dropped.
• Improved responsiveness of bursty background refreshes by coalescing rapid repeated triggers into a single run, cutting redundant work.
• Fixed a crash when triggering Authenticate on a remote MCP server in the /mcp panel.
• Fixed MCP tool results containing embedded resources being silently dropped, so text and inline media returned by MCP servers now surface in the conversation.
• Fixed permission checks splitting a single command into a pipeline when an argument contained quoted shell metacharacters (such as --grep="a|b"), which caused spurious permission prompts.
• Fixed the file-view and file-search tools failing with invalid-UTF-8 errors when a multi-byte character was split at a truncation boundary.
• Fixed a data race when collecting customization rules by guarding the shared structures.