docs: show custom keymaps in mail picker prompt title (#22)

[monkeyxite]

Closes #22

Mirrors the pattern already used by the calendar picker in lua/nvim-mail/calendar.lua (line 169): embed the picker's hotkey hints directly in prompt_title so users can discover the mappings without reading the source or hunting for Telescope's <C-/> help popup.

Before

  Notmuch Search

After

  Notmuch  ·  C-r:reply-all  C-o:browser  C-t:tag  C-y:msgid  C-l:preview

No behavior change, cosmetic only. All existing telescope tests still pass.

[gemini-code-assist]

Code Review

This pull request updates the prompt title in the Telescope mail extension to display helpful keybinding shortcuts. Feedback suggests correcting the label for the C-r keybinding from reply-all to reply to accurately reflect its actual implementation and prevent user confusion.

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[gemini-code-assist]

The prompt title labels <C-r> as reply-all. However, looking at the implementation of the <C-r> mapping (lines 74-125), it only extracts the From header and populates the To field of the reply draft, without including other recipients (like Cc). Therefore, it functions as a standard reply rather than a reply-all. Labeling it as reply would be more accurate and prevent user confusion.

    prompt_title = '  Notmuch  ·  C-r:reply  C-o:browser  C-t:tag  C-y:msgid  C-l:preview',

[monkeyxite]

Bot noted that <C-r> on main is not reply-all — correct against main's current state. This label becomes accurate once #18 (reply-all feature) merges. Not fixing here; the label will be correct via merge order.

If #18 is closed instead of merged, this PR's label should switch back to C-r:reply.