fix: remove pnpm cache from bump-npm-version (#60)

## Summary

`bump-npm-version.yaml` never runs `pnpm install`, so `setup-node` with
`cache: pnpm` fails at teardown because the cache directory doesn't
exist:

```
Path Validation Error: Path(s) specified in the action for caching do(es) not exist, hence no cache is being saved.
```

This caused the `Post Setup Node.js` step to fail in
datum-cloud/activity#160.

## Changes

- Remove `pnpm/action-setup` step — not needed when there's no install
- Remove `cache: pnpm` and `cache-dependency-path` from `setup-node` —
same reason
- Remove `lockfile-path` input — only existed to support pnpm caching

The workflow only needs Node.js to read `package.json` (version
resolution) and run `npm version` (bump + tag).

## Test plan

- [ ] Merge and tag `v1.13.1`
- [ ] Re-run the failing workflow in datum-cloud/activity — `Post Setup
Node.js` should no longer fail

Author: kevwilliams

.github/workflows/bump-npm-version.yaml

@@ -20,14 +20,6 @@ on:
         type: number
         description: "Node.js version to use"
         default: 24
-      lockfile-path:
-        required: false
-        type: string
-        description: >
-          Path to pnpm-lock.yaml relative to the repo root.
-          Defaults to "<package-path>/pnpm-lock.yaml".
-          Set to "pnpm-lock.yaml" for monorepos with a root lockfile.
-        default: ""
     outputs:
       new-version:
         description: "The new version after bumping (e.g. `v1.3.0`)"
@@ -48,17 +40,10 @@ jobs:
         with:
           ref: main
 
-      - name: Setup pnpm
-        uses: pnpm/action-setup@v4
-        with:
-          package_json_file: ${{ inputs.package-path }}/package.json
-
       - name: Setup Node.js
         uses: actions/setup-node@v4
         with:
           node-version: ${{ inputs.node-version }}
-          cache: pnpm
-          cache-dependency-path: ${{ inputs.lockfile-path || format('{0}/pnpm-lock.yaml', inputs.package-path) }}
 
       - name: Resolve new version
         id: resolve-version