ops: automate the release process (release-please, subgraph tagging)

[BigLep]

Background

#49 identified that the release process was undocumented. #197 addresses that by adding a release issue template with a manual checklist.

This issue tracks the remaining automation work to reduce manual steps in that checklist.

Potential items to automate

• Integrate release-please — automate changelog generation, version bumping, and GitHub release creation from conventional commits
• Automate subgraph prod tagging — after indexing completes, automatically apply the prod tag to filecoin-pay-mainnet and filecoin-pay-calibration (e.g. via a workflow triggered by a Goldsky webhook or polling the indexing status)
• Add smoke-test CI step — run a basic query against the newly deployed subgraph URL before tagging as prod, so bad deploys don't reach production silently
• Announce on completion — post a Slack/Filecoin Slack notification to #fil-foc automatically when the prod tag is applied

References

• Release template: .github/ISSUE_TEMPLATE/release.md
• Deploy workflow: .github/workflows/deploy.yml
• Goldsky dashboard: https://app.goldsky.com/project_cmb9tuo8r1xdw01ykb8uidk7h/dashboard/subgraphs