feat(hook): add OpenCode TUI stats panel with rtk init installation

Description

RTK already has an OpenCode core plugin (rtk init -g --opencode) that hooks into tool execution to rewrite commands. What's missing is visibility into the token savings — users currently have to run rtk gain manually to see how much they're saving.
This feature adds a TUI stats panel for OpenCode that shows real-time token savings in the IDE sidebar, installed automatically alongside the core plugin.

Proposed behavior

TUI plugin (rtk-stats.tsx)

A sidebar widget (sidebar_content slot, order: 40) showing:

• Header: RTK version with GitHub release check (✓/⚠)
• Stats (expandable): total commands, input/output tokens, tokens saved (green), average savings rate
• Efficiency meter: visual progress bar (████████░░░░ 96.4%)
• Refresh: every 30 minutes (background info, not real-time monitor)
• Per-project badge: shows project name when project-scoped stats exist
• Graceful degradation: if RTK is not installed, shows "RTK not present" — never crashes the UI

Installation (rtk init -g --opencode)

The existing rtk init -g --opencode command should install both:

1. ~/.config/opencode/plugins/rtk.ts (core plugin — auto-rewrite) — already exists
2. ~/.config/opencode/tui-plugins/rtk-stats.tsx (TUI stats panel) — new
3. Auto-patch ~/.config/opencode/tui.json to register the TUI plugin — new
   Properties:

• Idempotent: re-running the command is a no-op if content is unchanged
• Graceful: if tui.json is missing → creates it; if malformed → rebuilds it
• Atomic writes via NamedTempFile + .persist()

Data sources

• rtk gain -f json — overall stats
• rtk gain --project -f json — project-scoped stats (when applicable)
• rtk gain --quota — quota information (text parsed)
• GitHub releases API — version check

Files changed

Estimated token savings

The panel itself doesn't save tokens (it's visual), but it surfaces the savings data that RTK already collects, making users more aware of the value. Reduces friction by removing the need to run rtk gain manually.

Implementation status

Already implemented and tested on a fork. Ready for review.

Environment

• RTK version: 0.28.x+
• OpenCode: any version with TUI plugin support

Start with the existing OpenCode installation path in src/hooks/init.rs and related constants in src/hooks/constants.rs, then inspect hooks/opencode/rtk-stats.tsx. Verify the described panel states and the rtk init -g --opencode installation behavior, including idempotent, malformed, missing, and atomic tui.json handling; done means the plugin is installed and registered without breaking when RTK is absent.