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/agentapi/subagent.go

@@ -71,7 +71,7 @@ func (a *SubAgentAPI) CreateSubAgent(ctx context.Context, req *agentproto.Create
 	// An ID is only given in the request when it is a terraform-defined devcontainer
 	// that has attached resources. These subagents are pre-provisioned by terraform
 	// (the agent record already exists), so we update configurable fields like
-	// display_apps rather than creating a new agent.
+	// display_apps and directory rather than creating a new agent.
 	if req.Id != nil {
 		id, err := uuid.FromBytes(req.Id)
 		if err != nil {
@@ -97,6 +97,16 @@ func (a *SubAgentAPI) CreateSubAgent(ctx context.Context, req *agentproto.Create
 			return nil, xerrors.Errorf("update workspace agent display apps: %w", err)
 		}
 
+		if req.Directory != "" {
+			if err := a.Database.UpdateWorkspaceAgentDirectoryByID(ctx, database.UpdateWorkspaceAgentDirectoryByIDParams{
+				ID:        id,
+				Directory: req.Directory,
+				UpdatedAt: createdAt,
+			}); err != nil {
+				return nil, xerrors.Errorf("update workspace agent directory: %w", err)
+			}
+		}
+
 		return &agentproto.CreateSubAgentResponse{
 			Agent: &agentproto.SubAgent{
 				Name:      subAgent.Name,

coderd/agentapi/subagent_test.go

@@ -1267,11 +1267,11 @@ func TestSubAgentAPI(t *testing.T) {
 					agentID, err := uuid.FromBytes(resp.Agent.Id)
 					require.NoError(t, err)
 
-					// And: The database agent's other fields are unchanged.
+					// And: The database agent's name, architecture, and OS are unchanged.
 					updatedAgent, err := db.GetWorkspaceAgentByID(dbauthz.AsSystemRestricted(ctx), agentID)
 					require.NoError(t, err)
 					require.Equal(t, baseChildAgent.Name, updatedAgent.Name)
-					require.Equal(t, baseChildAgent.Directory, updatedAgent.Directory)
+					require.Equal(t, "/different/path", updatedAgent.Directory)
 					require.Equal(t, baseChildAgent.Architecture, updatedAgent.Architecture)
 					require.Equal(t, baseChildAgent.OperatingSystem, updatedAgent.OperatingSystem)
 
@@ -1280,6 +1280,42 @@ func TestSubAgentAPI(t *testing.T) {
 					require.Equal(t, database.DisplayAppWebTerminal, updatedAgent.DisplayApps[0])
 				},
 			},
+			{
+				name: "OK_DirectoryUpdated",
+				setup: func(t *testing.T, db database.Store, agent database.WorkspaceAgent) *proto.CreateSubAgentRequest {
+					// Given: An existing child agent with a stale host-side
+					// directory (as set by the provisioner at build time).
+					childAgent := dbgen.WorkspaceAgent(t, db, database.WorkspaceAgent{
+						ParentID:        uuid.NullUUID{Valid: true, UUID: agent.ID},
+						ResourceID:      agent.ResourceID,
+						Name:            baseChildAgent.Name,
+						Directory:       "/home/coder/project",
+						Architecture:    baseChildAgent.Architecture,
+						OperatingSystem: baseChildAgent.OperatingSystem,
+						DisplayApps:     baseChildAgent.DisplayApps,
+					})
+
+					// When: Agent injection sends the correct
+					// container-internal path.
+					return &proto.CreateSubAgentRequest{
+						Id:        childAgent.ID[:],
+						Directory: "/workspaces/project",
+						DisplayApps: []proto.CreateSubAgentRequest_DisplayApp{
+							proto.CreateSubAgentRequest_WEB_TERMINAL,
+						},
+					}
+				},
+				check: func(t *testing.T, ctx context.Context, db database.Store, resp *proto.CreateSubAgentResponse, agent database.WorkspaceAgent) {
+					agentID, err := uuid.FromBytes(resp.Agent.Id)
+					require.NoError(t, err)
+
+					// Then: Directory is updated to the container-internal
+					// path.
+					updatedAgent, err := db.GetWorkspaceAgentByID(dbauthz.AsSystemRestricted(ctx), agentID)
+					require.NoError(t, err)
+					require.Equal(t, "/workspaces/project", updatedAgent.Directory)
+				},
+			},
 			{
 				name: "Error/MalformedID",
 				setup: func(t *testing.T, db database.Store, agent database.WorkspaceAgent) *proto.CreateSubAgentRequest {