feat: add navigation, help overlay, yank, CLI parsing and file history

[greenbugx]

Move the details/diff/files fetch onto a worker thread and add the
bindings users of this kind of tool expect.

Fetches were synchronous: opening the Diff pane on a large commit read
the entire patch into a Vec<String> on the UI thread, and every
selection change while a pane was open re-read it. DetailWorker now
does that work off-thread, with three separate guarantees: requests
coalesce (drain the queue, keep the last) so a keypress burst costs one
fetch; replies carry a sequence number so a slow fetch cannot repaint a
pane the user has already left; and cancel() stops a closed pane being
repopulated. The main loop polls on the tick it already had.

Navigation: g/G and Home/End for the top and bottom of the focused
pane, PageUp/PageDown by a screenful, and J/K to walk commits with a
pane open so a diff can be followed down the history.

Discoverability: ? opens a modal keymap overlay. The keymap is data in
ui::help, so the same table is rendered, measured, and checked by a
test that fails when main.rs handles a key the overlay omits. The
status bar had room for about eight bindings.

y sends the selected commit's full hash to the terminal via OSC 52,
which works over SSH and needs no dependency. The message reads "sent
to clipboard" because OSC 52 gives no acknowledgement.

Arguments are parsed before the terminal enters raw mode, so --help
prints to a normal screen instead of being handed to
Repository::discover as a path named "--help". Unknown flags are an
error with a non-zero exit rather than a mystery repository.

l in the changed-files pane scopes the graph to that path, Esc leaves.
file_history decides "touched" by comparing the blob oid at the path
against the first parent, matching git log -- <path>; the filtered list
is laid out with GraphEngine::build_linear because the graph engine
holds a lane open for parents that filtering removed, and would widen
into a staircase one lane per row.

Also fixes a pre-existing bug in the refs pane: it copied its ListState
by value before rendering, discarding the scroll offset the widget
wrote back, which pinned the pane to its first screenful.

[chatgpt-codex-connector]

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