docs: document remote-use keybindings for sessionizer flows

[andrewchng]

What

Document the remote-use gotcha for Sessionizer keybindings, learned from a real failure:

• Under herdr --remote <target>, keybindings may resolve from the launching machine rather than the remote server's config, so sessionizer.open / sessionizer.worktree-open can silently do nothing.
• The fix that works: attach with herdr --remote <target> --remote-keybindings server.
• Explicitly notes that bindings on the launching machine do not trigger the plugin in a remote session (verified: they were present locally and still failed).
• Also notes the plugin runs on the remote host — bun, fzf, and the plugin checkout must exist there.

Verification

• README-only change; full suite passes (152 pass / 0 fail).