Wire automatic F-key pass-through into private_dot_bash_env

[blooop]

Question

Apply the decided change. Blocked on validation — do not start until the prototype ticket
has cleared, in particular its mouse-scroll go/no-go.

Four edits, all in the chezmoi source dir (chezmoi source-path; never a hardcoded
/home/<user> path):

1. private_dot_bash_env — add the PS0/PROMPT_COMMAND block from the plugin-route
   ticket's resolution, placed after the pane-shell EXIT trap block. Keep every
   zellij action wrapped in timeout, matching that trap's convention and for the same
   stated reason. Match the surrounding comment density — this file explains why, at
   length, and a bare block would read as foreign.
2. dot_config/zellij/config.kdl.tmpl, the tmux block comment — it currently says the
   mode is "entered with Ctrl+; d", which is no longer the whole story.
3. dot_config/zellij/config.kdl.tmpl, the shared_except "tmux" comment — it names
   htop, Midnight Commander and nested Zellij as the casualties of the F-key hot path. It can
   now say they are handled automatically.
4. README.md cheatsheet — required by the repo's own rule. F-keys behave differently
   inside an ssh/htop pane now, which is user-visible even though no binding changed.

Two things settled elsewhere in this map that must not be re-litigated here:

• Do not change F12 to SwitchToMode "Pane". That was only needed to escape a
  level-triggered plugin.
• Do not add nvim to the trigger list. Autolock's Locked keeps F1–F11 so tab
  switching works from inside the editor; pass-through would take that away.

Verify with chezmoi apply --force and a fresh pane — the hooks do not reach existing
ones. Document the one known rough edge: returning to a pane where ssh is already running
still needs a manual Ctrl+; d.