feat: get a desktop notification when a watched build finishes

[enesccinar]

Waiting on a build meant keeping its log pane open, or coming back and
pressing r until it finished. Now a on a build — from the build list, a
pull request's gate runs, or the log pane — watches it, and boardwalk puts a
desktop notification up when it succeeds, fails, partially succeeds or is
canceled. Press a again to stop watching.

Related to #19. That issue asks for a watch on new pull requests; this is the
build half of the same idea, and the notifier is its own package so a pull
request watch — or a builds -watch subcommand that works without the TUI
open — can reuse it.

internal/notify. Send(title, message), built the way the clipboard and
browser actions are: a pure list of candidate commands per platform, first one
on PATH wins. macOS uses terminal-notifier when it is installed, otherwise
osascript; Linux uses notify-send; Windows says it is not supported. A
missing tool and a tool that refuses are reported differently.

The watch. Root owns it rather than a view, so a watch survives esc back
to the menu. Watched builds are checked every twenty seconds for as long as
boardwalk is open, and the header reads · watching N. A finished build is
dropped and announced — a failure names the first task that failed and its
first error, from the timeline — and the status line says the same. A check
that fails keeps the watch and retries next tick; a notifier that fails says
so on the status line. A build that has already finished is not watched.

Notes

a rather than n: n is next-error on the log pane the build list opens.
a is free on the build list, the log pane and the pull request pane p
opens. It is on the help panel, not the footer, so the short help still fits
at 80 columns.

The poll and the notifier run in a command, off the update loop, with the ids
copied out first so the command never reads the map Update writes.

Whether a macOS notification stays on screen is the app's alert style in
System Settings, not something the command can ask for — set terminal-notifier
(or Script Editor, for osascript) to Persistent to make them sticky.

go test ./... and go vet ./... clean.

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