feat(picker): key-hint footer and vim ? overlay #67

Part of #61.

What

Custom keys nobody remembers are dead keys, so the Picker shows them:

• Footer: one line under the Query listing the bound Actions, e.g. enter jump · ctrl+g remote · ctrl+o files · …, truncated to the terminal width. jump, or whatever holds enter, comes first; the rest follow in config order, with built-ins last.
• Vim overlay: in vim list focus, ? toggles a full overlay of every key, including navigation. The default key map has no free key for an overlay (? is typing), so it shows only the footer.
• [picker] hints = false hides the footer.
• The same footer line carries the one-line Action errors and confirmations from feat: Actions — run a command on the selected Project with a key #62, feat: built-in remote Action opens the git remote (ctrl+g) #65, feat: built-in copy Action copies the Project path (ctrl+y) #66 and feat(picker): forget the selected Project with ctrl+d #69; a message replaces the hints until the next key press.

Acceptance

• Truncation is view-tested at narrow and wide widths.
• The overlay is tested opening and closing in vim mode.
• picker.hints is in config and the example config.