feat(startup): enable startup clone requests from CLI and IPC

Adds startup clone support with a new clone request type and parsing.
It wires a CLI and IPC pathway to forward a clone to a running app.
UI and docs were updated to reflect startup clone behavior.

- Introduce StartupCloneRequest and argument parsing.
- Wire IPC to pass clone requests and clone on startup.
- UI updated to queue clone requests and trigger clone.

CHANGELOG.md

@@ -25,6 +25,10 @@ The project uses calendar-style versions in the form `YYYY.M.PATCH`.
   the Git index without deleting their working-tree contents.
 - Gitty can open a repository directly at startup through the `--repo PATH` or
   `--repo=PATH` command-line argument.
+- The executable can clone and immediately open a repository with
+  `clone REMOTE TARGET`, `--clone REMOTE TARGET`, or `--clone=REMOTE TARGET`.
+  Relative targets use the caller's working directory, and requests are also
+  forwarded to an already-running Gitty instance.
 
 ### Changed