feat(desktop): base-branch picker for new worktree dialog

The sidebar "New worktree" button branched off whatever HEAD you were
on — now a filterable Popover+Command combobox lets you pick any local
or remote-tracking branch as the base, defaulting to origin/HEAD.

Backend listBaseBranches() queries refs/heads + refs/remotes via
for-each-ref, flags origin/HEAD (falling back to the local default for
no-remote repos). Mirrored on the Python REST API side
(base_branch_list + /api/git/base-branches route).

hermes_cli/web_git.py

@@ -644,3 +644,46 @@ def branch_switch(cwd: str, branch: str) -> dict:
         raise RuntimeError("Branch name is required.")
     _git_ok(cwd, ["switch", target])
     return {"branch": target}
+
+
+def base_branch_list(cwd: str) -> list[dict]:
+    """Local heads + remote-tracking refs for the base-branch picker.
+
+    The remote default (origin/HEAD) is flagged so the UI can preselect it.
+    """
+    out = _git_out(
+        cwd,
+        [
+            "for-each-ref",
+            "--format=%(refname:short)\t%(committerdate:iso)",
+            "--sort=-committerdate",
+            "refs/heads",
+            "refs/remotes",
+        ],
+    )
+    if not out:
+        return []
+    remote_default = _git_out(
+        cwd, ["symbolic-ref", "--quiet", "--short", "refs/remotes/origin/HEAD"]
+    ).strip()
+    local_default = _default_branch(cwd) if not remote_default else ""
+    result: list[dict] = []
+    for line in out.split("\n"):
+        line = line.strip()
+        if not line:
+            continue
+        name = line.split("\t")[0]
+        result.append(
+            {
+                "name": name,
+                "isRemote": name.startswith("origin/"),
+                # origin/HEAD when a remote exists; otherwise the local
+                # default (main/master/init.defaultBranch) so a no-remote
+                # repo still flags its trunk.
+                "isDefault": bool(
+                    (remote_default and name == remote_default)
+                    or (not remote_default and local_default and name == local_default)
+                ),
+            }
+        )
+    return result