docs: add session-title, worktree-tools and backslash-newline plugins - #805

Adds three OpenCode plugins:

• opencode-session-title: TUI plugin that shows the session title, dimmed, on the agent/model row under the prompt, like Claude Code shows it on its input box. Updates live on rename and hides in very narrow windows.
• opencode-worktree-tools: worktree_create / worktree_list / worktree_remove tools in .claude/worktrees, where Claude Code puts them. Tools only, no prompt injection or new terminals. Different from opencode-worktree, which spawns terminals and syncs files.
• opencode-backslash-newline: \ then Enter inserts a newline in the prompt, like Claude Code.

npm run validate passes.