fix: update directory for terraform-managed subagents (#24220)

When a devcontainer subagent is terraform-managed, the provisioner sets
its directory to the host-side `workspace_folder` path at build time. At
runtime, the agent injection code determines the correct
container-internal
path from `devcontainer read-configuration` and sends it via
`CreateSubAgent`.

However, the `CreateSubAgent` handler only updated `display_apps` for
pre-existing agents, ignoring the `Directory` field. This caused
SSH/terminal
sessions to land in `~` instead of the workspace folder (e.g.
`/workspaces/foo`).

Add `UpdateWorkspaceAgentDirectoryByID` query and call it in the
terraform-managed subagent update path to also persist the directory.

Fixes PLAT-118

<details><summary>Root cause analysis</summary>

Two code paths set the subagent `Directory` field:

1. **Provisioner (build time):** `insertDevcontainerSubagent` in
`provisionerdserver.go`
   stores `dc.GetWorkspaceFolder()` — the **host-side** path from the
   `coder_devcontainer` Terraform resource (e.g. `/home/coder/project`).

2. **Agent injection (runtime):**
`maybeInjectSubAgentIntoContainerLocked` in
`api.go` reads the devcontainer config and gets the correct
**container-internal**
path (e.g. `/workspaces/project`), then calls `client.Create(ctx,
subAgentConfig)`.

For terraform-managed subagents (those with `req.Id != nil`),
`CreateSubAgent`
in `coderd/agentapi/subagent.go` recognized the pre-existing agent and
entered
the update path — but only called `UpdateWorkspaceAgentDisplayAppsByID`,
discarding the `Directory` field from the request. The agent kept the
stale
host-side path, which doesn't exist inside the container, causing
`expandPathToAbs` to fall back to `~`.

</details>

> [!NOTE]
> Generated by Coder Agents

coderd/database/querier.go

@@ -1011,6 +1011,7 @@ type sqlcQuerier interface {
 	UpdateWorkspace(ctx context.Context, arg UpdateWorkspaceParams) (WorkspaceTable, error)
 	UpdateWorkspaceACLByID(ctx context.Context, arg UpdateWorkspaceACLByIDParams) error
 	UpdateWorkspaceAgentConnectionByID(ctx context.Context, arg UpdateWorkspaceAgentConnectionByIDParams) error
+	UpdateWorkspaceAgentDirectoryByID(ctx context.Context, arg UpdateWorkspaceAgentDirectoryByIDParams) error
 	UpdateWorkspaceAgentDisplayAppsByID(ctx context.Context, arg UpdateWorkspaceAgentDisplayAppsByIDParams) error
 	UpdateWorkspaceAgentLifecycleStateByID(ctx context.Context, arg UpdateWorkspaceAgentLifecycleStateByIDParams) error
 	UpdateWorkspaceAgentLogOverflowByID(ctx context.Context, arg UpdateWorkspaceAgentLogOverflowByIDParams) error