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/__tests__/lanes-cli.test.js

@@ -458,9 +458,17 @@ describe("agenthub lanes add", () => {
   });
 
   it("reports the failure notes, not success, when provisioning fails", async () => {
-    const emptyRepo = fs.mkdtempSync(path.join(os.tmpdir(), "agenthub-cli-unborn-repo-"));
+    // A branch named `feat` makes the default `feat/<slug>` impossible to
+    // create, so provisioning fails inside git (an unborn repo is now refused
+    // up front with EEMPTYREPO and never reaches provisioning).
+    const emptyRepo = fs.mkdtempSync(path.join(os.tmpdir(), "agenthub-cli-clash-repo-"));
     try {
       await git(["init", "-b", "main"], emptyRepo);
+      await git(
+        ["-c", "user.email=t@h", "-c", "user.name=t", "commit", "--allow-empty", "-m", "i"],
+        emptyRepo
+      );
+      await git(["branch", "feat"], emptyRepo);
       const r = await cli(
         ["lanes", "add", "--repo", emptyRepo, "--title", "CLI Unborn"],
         emptyRepo