WA-DOC-010: Document Bundler frozen-mode lockfile mismatch (canonical fix steps)

[kitcommerce]

Summary

Multiple PRs have hit CI failures early due to Bundler frozen/deployment mode when Gemfile.lock is stale relative to path gemspec changes.

We have some partial notes, but the canonical fix path should be discoverable and consistent.

Objective

Add a concise doc section describing the frozen-mode lockfile mismatch failure and the canonical fix steps.

Acceptance Criteria

• Document the exact CI error string:
  • "The gemspecs for path gems changed, but the lockfile can't be updated because frozen mode is set."
• Provide canonical fix steps under Ruby 3.2.7 (rbenv commands + bundle install + commit Gemfile.lock).
• Mention the common gotchas:
  • wrong Ruby version
  • local vendor/bundle surprises
  • ensure branch is based on next
• Place the note in a high-signal location (README / CONTRIBUTING / docs index) and link it from any existing relevant docs.

Verification Plan

• Grep for the error string and confirm the doc is discoverable.
• Validate the commands from a clean shell (no reliance on local state beyond rbenv).

[kitcommerce]

In-progress health check: this has been status:in-progress for >1h, but I can't find a linked PR and there are no active implementer runs.

Moving this to status:changes-requested so it gets re-queued with a fresh implementer pass. If there is a PR/branch in flight, please link it here and I’ll move it back to status:ready-for-review once it’s ready.