`coder_devcontainer`: sub-agent name collides when using `for_each` / `count`; no way to override

### Summary

The Terraform-managed sub-agent created for a `coder_devcontainer` resource derives its name from the Terraform *block name* only. When iterating with `for_each` or `count`, every instance shares the same block name, so all sub-agents try to claim the same workspace agent name and workspace creation fails with:

```
insert devcontainer "repos" subagent: insert subagent:
pq: workspace agent name "repos" already exists in this workspace build
```

There is no attribute on `coder_devcontainer` to override the sub-agent name from Terraform.

### Reproduction

Plan/apply this template with Coder server 2.25+ and `terraform-provider-coder` 2.14.0+:

```hcl
locals {
repos = toset(["repo-a", "repo-b", "repo-c"])
}

resource "coder_devcontainer" "repos" {
for_each = data.coder_workspace.me.start_count > 0 ? local.repos : toset([])
agent_id = coder_agent.dev[0].id
workspace_folder = "/home/user/workspaces/${each.value}"
config_path = "/home/user/workspaces/${each.value}/.devcontainer/devcontainer.json"
}
```

Result: workspace build fails because all three sub-agents are named `"repos"`.

### Root cause

In `coder/coder`'s provisioner, the sub-agent name is taken from `resource.Name`, which is the Terraform block name and does not include the `for_each` key:

https://github.com/coder/coder/blob/main/provisioner/terraform/resources.go#L466-L471

```go
agent.Devcontainers = append(agent.Devcontainers, &proto.Devcontainer{
Id: attrs.ID,
Name: resource.Name, // no for_each key suffix
WorkspaceFolder: attrs.WorkspaceFolder,
...
})
```

The `coder_devcontainer` provider schema has no `name` attribute to override this:

https://github.com/coder/terraform-provider-coder/blob/main/provider/devcontainer.go

### Workarounds

1. Define each devcontainer with a unique resource block name instead of iterating. Does not scale.
2. Wrap `coder_devcontainer` in a module and call it once per repo. Scales, but introduces a module layer purely to work around this.
3. Set `customizations.coder.name` in each `devcontainer.json`. Requires a PR to every repository and leaks deployment concerns into the repo.

### Proposal

Add an optional `name` attribute to `coder_devcontainer`:

```hcl
resource "coder_devcontainer" "repos" {
for_each = local.repos
name = each.value # new
agent_id = coder_agent.dev[0].id
workspace_folder = "/home/user/workspaces/${each.value}"
config_path = "..."
}
```

When set, `name` takes precedence over the block name when the provisioner constructs the `proto.Devcontainer`. This mirrors the `customizations.coder.name` escape hatch on the `devcontainer.json` side but keeps the override in Terraform, where the rest of the iteration already lives.

### Environment

- Coder server: `v2.32.0`
- Coder CLI / agent: `v2.31.6+f765029`
- `terraform-provider-coder`: `v2.14.0`

---
Created on behalf of manuel.anda@torc