Expose permission_mode in statusLine stdin JSON

What

Add the current permission mode (e.g. default, acceptEdits, plan, bypassPermissions) to the JSON passed to statusLine commands on stdin — e.g. a top-level permission_mode field.

Why

Custom status lines already render most live session state from the stdin JSON — model, effort/thinking, vim mode (.vim.mode), context usage, rate limits. Permission mode is one of the few pieces not exposed, so a custom statusLine can't show which mode the session is in. Exposing it lets users place the indicator wherever it fits their own layout.

Proposed shape

{
  "permission_mode": "acceptEdits",
  ...
}

Mirrors the existing .vim.mode / .model / .effort fields. Read-only — no change to the built-in indicator's behavior.

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Re-file of #54032, which was auto-closed by the stale bot for inactivity ("Please open a new issue if this is still relevant"). Related: #46419.