fix(ci): pass GITHUB_TOKEN to tagpr

[ryuichi1208]

Summary

The first main push after merging #1 triggered the tagpr workflow,
which failed immediately with:

git [config remote.origin.url]
https://github.com/ryuichi1208/rw
GitHub token not found
Error: Process completed with exit code 1.

Songmu/tagpr reads the GitHub token from the GITHUB_TOKEN
environment variable, not from the workflow's permissions: block
(which only controls what the auto-generated token can do, not
whether it's exposed to the action). This PR plumbs the token
explicitly:

- uses: Songmu/tagpr@v1
  env:
    GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

Test plan

• Once merged, the next main push triggers tagpr and it
  successfully opens a release PR (instead of erroring on the
  missing token).
• The release PR shows the next intended version (likely
  v0.0.1 since no tag exists yet) and a changelog derived
  from the commits since Initial commit.

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