Slice 9: git_checkout and create_branch (menu + MCP)

git boundary Checkout/CreateBranch; service GitCheckout/GitCreateBranch (activity detail names the branch; gitAction takes an ok-detail). HTTP /api/repo/git ops checkout + create-branch (branch field). MCP tools git_checkout and create_branch. <repo-menu> gains Switch branch… and New branch… (prompt for name). Service test covers create+switch and existing-branch failure. Checkout is not destructive - git refuses if it would overwrite changes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

components/repo-menu/repo-menu.js

@@ -12,6 +12,8 @@ const ITEMS = [
   { cmd: 'push',    label: 'Publish',               hint: 'push' },
   { cmd: 'fetch',   label: 'Check for updates',     hint: 'fetch' },
   { cmd: 'commit',  label: 'Save my work…',         hint: 'commit' },
+  { cmd: 'checkout', label: 'Switch branch…',       hint: 'checkout' },
+  { cmd: 'newbranch', label: 'New branch…',         hint: 'branch' },
   { sep: true },
   { cmd: 'active',  label: 'Set as active project' },
   { cmd: 'handoff', label: 'Ask Claude to switch here' },
@@ -91,6 +93,16 @@ class RepoMenu extends HTMLElement {
         if (msg && msg.trim()) await this.#git('commit', { message: msg });
         break;
       }
+      case 'checkout': {
+        const b = window.prompt(`Switch ${name} to which existing branch?`);
+        if (b && b.trim()) await this.#git('checkout', { branch: b.trim() });
+        break;
+      }
+      case 'newbranch': {
+        const b = window.prompt(`New branch name in ${name}:`);
+        if (b && b.trim()) await this.#git('create-branch', { branch: b.trim() });
+        break;
+      }
       case 'discard': {
         const ok = window.confirm(
           `Discard ALL uncommitted changes in ${name}?\n\n` +