refactor: eliminate getCheckoutStatusLite, use WorkspaceGitService

Route all git checkout queries through WorkspaceGitService instead of
shelling out to git on every call. This makes fetch_agents_request
instant when warm (cached in-memory snapshots) and deduplicates
concurrent cold-start refreshes via ensureWorkspaceTarget.

- buildProjectPlacementForCwd now uses workspaceGitService.getSnapshot()
- worktree-session call sites use WorkspaceGitService
- Deleted getCheckoutStatusLite and its 4 return types from checkout-git
- Fixed cold-start thundering herd: getSnapshot deduplicates via
  ensureWorkspaceTarget instead of spawning parallel git processes
- Extracted checkout-diff logic into WorkspaceGitService

packages/server/src/utils/checkout-git.test.ts

@@ -22,7 +22,6 @@ import {
   getCheckoutShortstat,
   getPullRequestStatus,
   getCheckoutStatus,
-  getCheckoutStatusLite,
   listBranchSuggestions,
   mergeToBase,
   mergeFromBase,
@@ -164,15 +163,6 @@ const x = 1;
     expect(removedLine?.tokens).toEqual([{ text: "old comment line", style: "comment" }]);
   });
 
-  it("returns lightweight checkout status for normal repos", async () => {
-    const status = await getCheckoutStatusLite(repoDir);
-    expect(status.isGit).toBe(true);
-    expect(status.currentBranch).toBe("main");
-    expect(status.worktreeRoot).toBe(repoDir);
-    expect(status.isPaseoOwnedWorktree).toBe(false);
-    expect(status.mainRepoRoot).toBeNull();
-  });
-
   it("exposes hasRemote when origin is configured", async () => {
     const remoteDir = join(tempDir, "remote.git");
     execSync(`git init --bare -b main ${remoteDir}`);
@@ -380,22 +370,6 @@ const x = 1;
     expect(message).toBe("worktree update");
   });
 
-  it("returns lightweight checkout status for .paseo worktrees", async () => {
-    const result = await createWorktree({
-      branchName: "main",
-      cwd: repoDir,
-      baseBranch: "main",
-      worktreeSlug: "lite-alpha",
-      paseoHome,
-    });
-
-    const status = await getCheckoutStatusLite(result.worktreePath, { paseoHome });
-    expect(status.isGit).toBe(true);
-    expect(status.worktreeRoot).toBe(result.worktreePath);
-    expect(status.isPaseoOwnedWorktree).toBe(true);
-    expect(status.mainRepoRoot).toBe(repoDir);
-  });
-
   it("returns mainRepoRoot pointing to first non-bare worktree for bare repos", async () => {
     const bareRepoDir = join(tempDir, "bare-repo");
     execSync(`git clone --bare ${repoDir} ${bareRepoDir}`);