fix: pre-landing review fixes (pass 4)

The pack's staged-blob list comes from the copied index, holds only blobs
staged against HEAD (not every tracked blob), skips gitlinks whose commit
the superproject lacks, and dies with its own message when pack-objects
fails. The live-checkout test covers a gitlink entry and a valueless
remote key replayed as true. CHANGELOG names the staged-blob carry.

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

test/ubicloud-runner.test.ts

@@ -340,9 +340,11 @@ test('pack ships a linked worktree with a self-contained .git on the checked-out
       const broken = Bun.spawnSync(['bash', '-c', `rm -f .git/index && TMPDIR=../tmpd ${pack} > ../pack-broken.tgz; echo rc=$?; ls ../tmpd`], { cwd: src, timeout: 60_000 });
       expect(broken.stderr.toString()).toContain('.git/index');
       expect(broken.stdout.toString().trim()).toBe('rc=1');
-      sh('git reset -q', src);
+      // A gitlink in the index (a submodule commit this object store lacks) is not packed, and a valueless
+      // remote key (`[remote "origin"] mirror`) replays as true.
+      sh('git reset -q && git update-index --add --cacheinfo 160000,1111111111111111111111111111111111111111,sub && printf \'[remote "origin"]\\n\\tmirror\\n\' >> .git/config', src);
       sh('mkdir vm2 && ' + pack.replace('pack .', 'pack src') + ' | tar -xzf - -C vm2', d);
-      expect(sh('git rev-parse --abbrev-ref @{u}', join(d, 'vm2')).trim()).toBe('origin/main');
+      expect(sh('git rev-parse --abbrev-ref @{u} && git config --bool --get remote.origin.mirror && git ls-files -s sub | cut -c1-6', join(d, 'vm2')).split('\n').filter(Boolean)).toEqual(['origin/main', 'true', '160000']);
     } finally { rmSync(d, { recursive: true, force: true }); }
   }, 90_000);