[codex] add agentbox devbox helpers

[dpunj]

What changed

• Adds an agentbox/ guide for a Tailscale SSH devbox with a canonical ~/code/versa-ts clone and per-task worktrees.
• Adds agentbox-doctor to check local readiness for git, GitHub CLI, Fish, tmux, Tailscale, Bun, Node, repo paths, auth, and Tailscale connectivity.
• Adds agent-worktree to create isolated ~/code/worktrees/versa-ts-agent-* workspaces from origin/main.
• Wires the helpers into bootstrap.fish and PATH documentation.
• Quiets fnm startup noise in Fish shell initialization.

Why

This gives the remote agent machine a small, inspectable setup path: warm repo access, persistent sessions through tmux, Tailscale-only SSH, and isolated git workspaces for agent edits.

Validation

• fish --no-config -n bin/agent-worktree
• fish --no-config -n bin/agentbox-doctor
• fish -n bootstrap.fish
• fish -n fish/config.fish