Implement Tag-Based Subgraph Deployment

## Done Criteria

Tagging and pushing a commit with changes to the subgraph code triggers an automated deployment of the subgraph to Goldsky.

I'd say automatic deployments should only be done to calibration first and then with a manual deployment trigger be rolled out to mainnet.

Documentation should be updated as well.

## Why Important

A manual deployment process is error prone and carries mental overhead that can be avoided with an automated pipeline.

## Notes

I expect the subgraph code to change much less frequently than, e.g., the dealbot backend, so when prioritising this issue, gauge the upside against the effort of implementing this.

However, we already have an existing workflow which may be used almost as drop-in: [filecoin-pay-explorer's `deploy.yml`](https://github.com/FilOzone/filecoin-pay-explorer/blob/main/.github/workflows/deploy.yml)

## References

- https://github.com/FilOzone/dealbot/pull/542#discussion_r3299526163
- [filecoin-pay-explorer's `deploy.yml`](https://github.com/FilOzone/filecoin-pay-explorer/blob/main/.github/workflows/deploy.yml)