chore: move agent's file API into separate package (#21531)

This makes it so we can test it directly without having to go through
Tailnet, which appears to be causing flakes in CI where the requests
time out and never make it to the agent.

Takes inspiration from the container-related API endpoints.

Would probably make sense to refactor the ls tests to also go through
the API (rather than be internal tests like they are currently) but I
left those alone for now to keep the diff minimal.

agent/api.go

@@ -27,6 +27,8 @@ func (a *agent) apiHandler() http.Handler {
 		})
 	})
 
+	r.Mount("/api/v0", a.filesAPI.Routes())
+
 	if a.devcontainers {
 		r.Mount("/api/v0/containers", a.containerAPI.Routes())
 	} else if manifest := a.manifest.Load(); manifest != nil && manifest.ParentID != uuid.Nil {
@@ -49,10 +51,6 @@ func (a *agent) apiHandler() http.Handler {
 
 	r.Get("/api/v0/listening-ports", a.listeningPortsHandler.handler)
 	r.Get("/api/v0/netcheck", a.HandleNetcheck)
-	r.Post("/api/v0/list-directory", a.HandleLS)
-	r.Get("/api/v0/read-file", a.HandleReadFile)
-	r.Post("/api/v0/write-file", a.HandleWriteFile)
-	r.Post("/api/v0/edit-files", a.HandleEditFiles)
 	r.Get("/debug/logs", a.HandleHTTPDebugLogs)
 	r.Get("/debug/magicsock", a.HandleHTTPDebugMagicsock)
 	r.Get("/debug/magicsock/debug-logging/{state}", a.HandleHTTPMagicsockDebugLoggingState)