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

[hammadmajid]

Closes #67. Part of #61.

What changed

• The footer's keys line is built from the resolved Actions instead of a hard-coded "enter jump": the Action on enter first, the rest in order, unbound Actions left out. Whole hints that do not fit are dropped behind a ….
• action.Merge now takes the table order and returns the user's own Actions in config.toml order, followed by the built-ins (it used to put built-ins first, then user Actions sorted by name). config reads that order from the raw [actions.*] headers.
• A message (error or confirmation) still replaces the hints until the next key; its styling moved to one messageLine helper next to the hints.
• Vim list focus: ? opens an overlay of every key (navigation and Actions), and ?, esc or q close it. While it is open other keys do nothing, ctrl+c still cancels. The footer shows ? help in vim list focus. An Action bound to ? keeps the key. The default key map has no overlay.
• [picker] hints (default true) in config; false hides the hints, leaving the match count and messages. config.example.toml does not exist yet, so it is not updated.
• README documents all of it.

Verification

• gofmt -l . empty, go vet ./... clean, golangci-lint run ./... 0 issues, binary builds, no file over 300 lines.
• go test ./... passes except the known local-only fish cases of TestScriptExecution.
• New tests: footer at wide and narrow widths (and never wider than the terminal for widths 5-80), hint order, message replacing hints, hints = false, the overlay opening and closing in vim mode, only in list focus, fitting heights 3-40; config default/parse/type error; action order.