feat(lanes,git): publish a repo with no remote, and read a repo with no commits

What broke: a lane on a fresh `git init` repo had no working git surface.
`statusCounts` ran `rev-parse --short HEAD` unguarded, so `gitFacts` threw
and `GET /api/lanes/:id/git` answered `{available:false}` — the Changes
panel showed no header, and `commit`/`switch` 500'd through `isDirty`.
Add lane in Worktree mode accepted such a repo and left a `failed` row
behind, and a repo with commits but no remote had a permanently disabled
Push with no way to give it one short of a terminal.

What changed:
- worktree.js: an unborn HEAD is a normal state. `statusCounts` reports
  `head: null`; `gitFacts` reads the branch via `symbolic-ref`, an empty
  subject, and a new `unborn` field.
- git-ops.js: an 11th write, `publish` — `remote add origin -- <url>` then
  `push -u origin HEAD`, only with a commit (ENOCOMMITS) and NO remote at
  all (EREMOTEEXISTS, so it never re-points one), the url allowlisted by
  `validateRemoteUrl` (https without userinfo, ssh://, scp-like; refuses
  local paths, file://, ext::, fd::, git://, http://, leading `-`:
  EBADURL), and `origin` removed again when the push fails.
- POST /api/lanes/worktree answers 400 EEMPTYREPO for a repo with no
  commits instead of creating a row that can only fail.
- LaneGitBar: unborn repos show "no commits yet" and a commit-first hint;
  no remote offers Publish… with an inline URL box; a remote but no
  upstream relabels Push as "Publish branch". AddLaneModal disables Add
  on an empty repo and offers "Use as a Repo lane".
- The two tests that used an unborn repo to force a provisioning failure
  now use a `feat` ref clash, since that repo is refused up front.
- CLAUDE.md rule reworded to "the boring eleven" with publish's guards;
  docs/API.md, docs/LANES.md, ARCHITECTURE.md, server/README.md, openapi.

client/src/components/lanes/__tests__/AddLaneModal.test.tsx

@@ -305,6 +305,28 @@ describe("AddLaneModal — worktree mode (default)", () => {
     expect(screen.getByRole("button", { name: "Add lane" })).toBeDisabled();
   });
 
+  it("refuses a worktree of a repo with no commits and steers to Repo mode", async () => {
+    vi.mocked(api.lanes.branches).mockResolvedValue({
+      branches: [],
+      current: "main",
+      worktrees: [],
+    });
+    renderModal();
+    const user = userEvent.setup();
+
+    const repoField = screen.getByLabelText("Source repository");
+    await focusField(user, repoField);
+    await user.type(repoField, "/Users/tester/projects/empty");
+    expect(await screen.findByText(/commit first, then publish/i)).toBeInTheDocument();
+    await user.type(screen.getByLabelText("Title"), "demo");
+    // There is nothing to branch from, so the worktree cannot be created.
+    expect(screen.getByRole("button", { name: "Add lane" })).toBeDisabled();
+
+    await user.click(screen.getByRole("button", { name: "Use as a Repo lane" }));
+    expect(screen.getByRole("button", { name: "Repo" })).toHaveAttribute("aria-pressed", "true");
+    expect(screen.getByRole("button", { name: "Add lane" })).toBeEnabled();
+  });
+
   it("submits through the worktree provisioning endpoint, with the typed branch name", async () => {
     vi.mocked(api.lanes.branches).mockResolvedValue({
       branches: ["main"],