chore(terraform): migrate project_name default from "game-servers-poc" to "hyveon"

[CoderCoco]

Follow-up from the Hyveon rebrand (#212). Tracking the deferred half of the brand migration.

Scope

Change terraform/variables.tf's project_name default from \"game-servers-poc\" to \"hyveon\" (or another approved value), then plan and apply the resulting infrastructure churn.

Why this is a separate PR

`project_name` is interpolated into resource names, IAM identifiers, log group names, tag values, and Discord secret ARNs. Changing it isn't a text replacement — it forces AWS to:

• Re-create resources whose name is not in-place editable (CloudWatch log groups, Secrets Manager secrets, possibly ECS task definitions).
• Re-tag every resource that carries `Project = var.project_name`.
• Break the Cost Explorer 'Project' tag activation that's already configured in AWS Billing — the activation is per-tag-value, so a new value needs a fresh activation and historical cost data won't carry across.

So this needs a dedicated planning pass, not an inline edit during a rebrand.

Checklist

• Decide the new `project_name` value (`hyveon` is the obvious default; confirm).
• Read `terraform plan` output and enumerate every resource that will be replaced vs. retagged.
• Coordinate downtime if any in-flight game-server task definitions need to be redeployed.
• After apply, re-activate the new `Project = hyveon` cost allocation tag in AWS Billing → Cost allocation tags. Historical `game-servers-poc` cost data stays under the old activation.
• Update CLAUDE.md's reference to `Project=game-servers-poc` to the new value.
• Update `terraform/terraform.tfvars.example` if it sets the variable.
• Verify Discord Secrets Manager secrets came back online with the new ARN paths (the followup Lambda reads them at cold-start).
• Confirm the watchdog Lambda still finds tasks by tag.
• Sanity-check Cost Explorer breakdowns the next day.

Out of scope

The `@gsd/` → `@hyveon/` workspace scope rename — tracked separately in #142.