docs: note apm 0.27.0 --frozen regression and 0.25.0 pin workaround

[nobv]

Summary

• just apm sync / just apply fail on apm CLI 0.27.0 because --frozen validation regresses for full-repo claude_skill dependencies without their own apm.yml (e.g. currents-dev/playwright-best-practices-skill): version resolves to unknown, then --frozen looks for a per-package manifest that doesn't exist and errors.
• Plain apm install -g (no --frozen) still succeeds; only the CI-safe frozen check is affected. Relocking the lockfile with apm lock under 0.27.0 does not fix it.
• Documented the pin workaround (downgrade Homebrew's apm to 0.25.0 via the tap's formula history, brew pin) in the Troubleshooting section so future recurrences are quick to resolve.

Test plan

• just nix check passes
• Verified locally that pinning apm at 0.25.0 makes just apm sync succeed again