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.

server/routes/lanes.js

@@ -978,6 +978,9 @@ router.post("/:id/git/:action", sameOriginGuard, async (req, res) => {
       "ESUBDIR",
       "EBADSIDE",
       "EMARKERS",
+      "EBADURL",
+      "EREMOTEEXISTS",
+      "ENOCOMMITS",
     ];
     // A numeric code is git's own exit status (execFile reports one that way):
     // git ran and refused — a rejected push, a failed credential, a diverged
@@ -1054,6 +1057,24 @@ router.post("/worktree", sameOriginGuard, async (req, res) => {
     });
   }
 
+  // A repo with no commit has nothing a worktree could branch from: git would
+  // fail later, in the background job, and leave a `failed` row behind. Refuse
+  // it here instead — that repo is a Repo lane until its first commit.
+  if (!(await listBranches(sourceRepo)).branches.length) {
+    const unborn = await git(sourceRepo, ["rev-parse", "--verify", "HEAD"]).then(
+      () => false,
+      () => true
+    );
+    if (unborn) {
+      return res.status(400).json({
+        error: {
+          code: "EEMPTYREPO",
+          message: "sourceRepo has no commits yet — add it as a Repo lane and commit first",
+        },
+      });
+    }
+  }
+
   let slug;
   try {
     slug = slugify(body.slug || body.title);