feat: integrate new agentexec pkg (#15609)

- Integrates the `agentexec` pkg into the agent and removes the
legacy system of iterating over the process tree. It adds some linting
rules to hopefully catch future improper uses of `exec.Command` in the package.

cmd/coder/main.go

@@ -1,12 +1,20 @@
 package main
 
 import (
+	"fmt"
+	"os"
 	_ "time/tzdata"
 
+	"github.com/coder/coder/v2/agent/agentexec"
 	"github.com/coder/coder/v2/cli"
 )
 
 func main() {
+	if len(os.Args) > 1 && os.Args[1] == "agent-exec" {
+		err := agentexec.CLI()
+		_, _ = fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
 	var rootCmd cli.RootCmd
 	rootCmd.RunWithSubcommands(rootCmd.AGPL())
 }