new: port the shell, empty, makefile and go templates to manifests

Move the four templates' static file bodies into .tpl files under
data/templates/<id>/, referenced by their manifests' files: lists —
the shell skeleton script (executable, {command}-named) with its
skeleton-only debian/install mapping, the empty template's stub README,
the makefile hello.c/Makefile skeleton with its skeleton-only install
mapping, and go's go.mod/main.go skeleton (the go directive of go.mod
stays a literal: nothing about it is answer-derived).

The empty template ends up hookless — zero Rust, its registry entry
points at no hooks — and src/new/templates/empty.rs is deleted. The
shell and go hooks shrink to their probes (plus go's {go_import_path}
context value); the makefile hooks keep only the existing-tree hint
probing the packaged Makefile for a phony install: target, since that
heuristic reads the tree and cannot be data.

data/templates/go/go.mod.tpl

@@ -0,0 +1,3 @@
+module {name}
+
+go 1.21

data/templates/go/main.go.tpl

@@ -0,0 +1,8 @@
+// Placeholder for {name}, generated by `pkh new`.
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("Hello from {command}!")
+}

data/templates/go/manifest.yml

@@ -1,6 +1,8 @@
 ## The `go` template: a Go module built through dh-golang. The logic half
 ## — the go.mod module-line probe and the `{go_import_path}` value below —
-## lives in src/new/templates/go.rs.
+## lives in src/new/templates/go.rs; the skeleton bodies are static data
+## (the `go` directive of go.mod stays a literal: nothing about it is
+## answer-derived, so it has no {placeholder}).
 ##
 ## Schema: see src/new/templates/mod.rs.
 
@@ -15,4 +17,8 @@ architecture: any
 rules_dh_line: "dh $@ --buildsystem=golang"
 source_fields:
   XS-Go-Import-Path: "{go_import_path}"
-files: []
+files:
+  - path: go.mod
+    template: go.mod.tpl
+  - path: main.go
+    template: main.go.tpl