chore(showcase/harness): remove stray npm lockfile

`showcase/harness` is a pnpm workspace member (pnpm-workspace.yaml), so its
only install path is pnpm from the root `pnpm-lock.yaml`. Both consumers run
exactly that: `showcase/harness/Dockerfile` (`pnpm install --frozen-lockfile
--filter @copilotkit/showcase-harness...`) and the `harness unit suite` job
in `test_unit-showcase.yml`. Nothing anywhere ran `npm ci` in this directory
— no Dockerfile, workflow, or script referenced the lockfile.

Because no gate ever read it, it rotted unobserved. Against a fresh
regeneration it was 106 package versions stale, carried 51 packages no longer
required, and was missing `axe-core` outright — which is how it surfaced:
`npm ci` here failed with "Missing: axe-core@4.11.1 from lock file" for
anyone who saw the file and reasonably concluded this package installs with
npm.

Regenerating it was the wrong fix. npm resolves a materially different tree
than the one CI and prod actually run — 9 direct deps diverged, with
`@hono/node-server` off by a whole major (pnpm 2.0.0 vs npm 1.19.17) — and
npm cannot see the root `pnpm.overrides` block, 73 minimum-version floors
with several security patches among them, because that field is pnpm-only.
A working `npm ci` here would hand a developer a green install against
versions that never ship, and could silently resolve below an intended
security floor. The stale file was a signpost pointing the wrong way, so it
is removed rather than maintained.

The root `pnpm-lock.yaml` already resolves `axe-core` at 4.11.1 correctly;
verified that `pnpm install --frozen-lockfile --ignore-scripts` succeeds and
that axe-core 4.11.1 is present in the harness tree with no npm lockfile,
and that the harness typechecks clean once the gitignored showcase fixtures
are generated the way CI generates them.

Adds a NOTE in pnpm-workspace.yaml recording why no npm lockfile belongs
here, so the file is not re-added in good faith as it was in 671cc6ae1d.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

pnpm-workspace.yaml

@@ -36,6 +36,7 @@ packages:
   - "examples/teams"
   - "!examples/v1/_legacy"
   - "showcase/scripts"
+  # No npm lockfile here: installs with pnpm from the root pnpm-lock.yaml.
   - "showcase/harness"
   - "showcase/eval-webhook"
   - "showcase/angular"