fix: percent escaping for git --format on Windows (#629)

* test(server): add real-spawn round-trip for % in argv

RED test: pass --format=%(refname)... through spawnProcess to a child
node script that echoes argv[2]. Asserts the child receives the original
string verbatim. Wired into the Windows CI matrix to expose the
escapeWindowsCmdValue %-doubling bug under cmd.exe.

* test(server): use bare 'node' command so cmd.exe path is exercised

process.execPath has both an extension and a path separator on Windows,
so shouldUseWindowsShell returns false and the broken %-escape pipeline
is never invoked. Use the bare command name 'node' instead.

* fix(server): stop doubling % in cmd.exe arg escaping on Windows

cmd.exe only collapses `%%` → `%` inside batch files; on the command
line / via `cmd /c "..."` `%%` stays literal. Doubling `%` in
escapeWindowsCmdValue meant args like `--format=%(refname)` reached git
as `--format=%%(refname)`, which git interprets as the escape sequence
for a literal `%`, so format atoms appeared verbatim and the branch
picker showed `%(refname)%09%(committerdate:unix)` instead of branch
names.

Update unit tests that pinned the broken doubling behavior.

.github/workflows/ci.yml

@@ -122,6 +122,7 @@ jobs:
           npx vitest run
           src/utils/executable.test.ts
           src/utils/spawn.launch-regression.test.ts
+          src/utils/spawn.percent-escape.test.ts
           src/utils/spawn.test.ts
           src/utils/run-git-command.test.ts
           src/utils/checkout-git-rev-parse.test.ts