consider adding CLAUDE.md (or editor equivalent) at forest root

## Idea

When opening a forest root (or a tree) in Claude Code / Cursor / similar AI-aware editors, the agent has no context that it's inside a workforest. It doesn't know:

- the folder is a forest root, not a regular repo
- sibling directories are worktrees of the same repo on different branches
- how to navigate between trees, add new ones, etc.

It would be useful to drop a `CLAUDE.md` (and/or `.cursorrules`, `AGENTS.md`, etc.) at the forest root — or appended alongside an existing tree's `CLAUDE.md` — that briefly explains:

- this is a workforest layout
- each sibling directory is a git worktree for a branch
- pointers to `git forest add` / `list` / `remove`
- link to project docs

## Open questions

- forest-root file vs per-tree file (or both)?
- opt-in flag on `init` / `clone` / `migrate`, or always-on with opt-out?
- which agent files to write — `CLAUDE.md`, `AGENTS.md` (emerging standard), `.cursorrules`, all?
- if a tree already has a `CLAUDE.md`, append a workforest section rather than overwrite
- should it be a separate `git forest agent-init` command instead of bundled into setup?

## Why

Lets users open any tree (or the forest root) in an AI editor and have the agent immediately understand the layout without manual prompting.