feat: add app status tracking to the backend (#17163)

This does ~95% of the backend work required to integrate the AI work.

Most left to integrate from the tasks branch is just frontend, which
will be a lot smaller I believe.

The real difference between this branch and that one is the abstraction
-- this now attaches statuses to apps, and returns the latest status
reported as part of a workspace.

This change enables us to have a similar UX to in the tasks branch, but
for agents other than Claude Code as well. Any app can report status
now.

coderd/coderd.go

@@ -1228,6 +1228,7 @@ func New(options *Options) *API {
 				}))
 				r.Get("/rpc", api.workspaceAgentRPC)
 				r.Patch("/logs", api.patchWorkspaceAgentLogs)
+				r.Patch("/app-status", api.patchWorkspaceAgentAppStatus)
 				// Deprecated: Required to support legacy agents
 				r.Get("/gitauth", api.workspaceAgentsGitAuth)
 				r.Get("/external-auth", api.workspaceAgentsExternalAuth)