Fix CI dependency install step in test-and-build

[Copilot]

The test-and-build GitHub Actions job failed before typecheck/tests/build due to a broken dependency bootstrap command. The install step invoked a script path that required an existing Yarn node_modules state file and exited immediately.

• Root cause

  • CI used yarn install:all:immutable, which was resolved through script execution and failed with:
    • Couldn't find the node_modules state file - running an install might help (findPackageLocation)

• Workflow change

  • Replaced the single script invocation with explicit immutable installs at each workspace boundary:
    • root
    • server/
    • client/

• Net effect

  • CI now performs lockfile-enforced installs directly, avoiding script bootstrap behavior that depends on pre-existing install state.

- name: Install dependencies from lockfiles
  run: |
    yarn install --immutable
    yarn --cwd server install --immutable
    yarn --cwd client install --immutable