refactor(ci): drop registry-url, rely on project .npmrc for auth (#11)

## Summary

Refactor CI/deploy workflows to use the new `setup-vp` behavior from
[voidzero-dev/setup-vp#54](voidzero-dev/setup-vp#54).

- Drop `registry-url: https://npm.pkg.github.com` from all `setup-vp`
steps in `.github/workflows/ci.yml` and `.github/workflows/deploy.yml`.
- Rely on the repo-level `.npmrc`
(`@void-sdk:registry=https://npm.pkg.github.com`) plus `NODE_AUTH_TOKEN`
env: the action auto-generates a matching `_authToken` entry at
`$RUNNER_TEMP/.npmrc` for each registry declared in the project `.npmrc`
that doesn't already have one.
- The checked-in `.npmrc` stays minimal — no auth line needed.

## Pinning

Pinned to commit `af4ffd95e8c3d93e048fe8f03afeb2d4e3eba927` (head of PR
#54's `fix/respect-project-npmrc` branch) via a commented SHA. Bump back
to `@v1` once the PR merges and is released.

## Test plan

- [ ] `test` job on this PR runs `vp install` successfully (private
`@void-sdk/*` packages resolve from GitHub Packages)
- [ ] `staging-deploy` job on this PR completes `vpx void deploy`

Author: fengmk2

.github/workflows/ci.yml

@@ -14,10 +14,9 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v6
-      - uses: voidzero-dev/setup-vp@v1
+      - uses: voidzero-dev/setup-vp@4f5aa3e38c781f1b01e78fb9255527cee8a6efa6 # v1.8.0
         with:
           cache: true
-          registry-url: https://npm.pkg.github.com
         env:
           NODE_AUTH_TOKEN: ${{ secrets.NODE_AUTH_TOKEN }}
       - run: vp check
@@ -30,10 +29,9 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v6
-      - uses: voidzero-dev/setup-vp@v1
+      - uses: voidzero-dev/setup-vp@4f5aa3e38c781f1b01e78fb9255527cee8a6efa6 # v1.8.0
         with:
           cache: true
-          registry-url: https://npm.pkg.github.com
         env:
           NODE_AUTH_TOKEN: ${{ secrets.NODE_AUTH_TOKEN }}
       - run: vpx void deploy

.github/workflows/deploy.yml

@@ -9,10 +9,9 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v6
-      - uses: voidzero-dev/setup-vp@v1
+      - uses: voidzero-dev/setup-vp@4f5aa3e38c781f1b01e78fb9255527cee8a6efa6 # v1.8.0
         with:
           cache: true
-          registry-url: https://npm.pkg.github.com
         env:
           NODE_AUTH_TOKEN: ${{ secrets.NODE_AUTH_TOKEN }}
       - run: vpx void deploy