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/empty/README.tpl

@@ -0,0 +1 @@
+{name} - empty base tree scaffolded by `pkh new`; there is intentionally no upstream build system here.

data/templates/empty/manifest.yml

@@ -1,6 +1,9 @@
 ## The `empty` template: a metapackage (non-empty Depends list) or an
 ## empty base package with no build system at all — pure `dh $@` plumbing
-## as a starting point for hand-written rules.
+## as a starting point for hand-written rules. Pure data: no hooks, the
+## metapackage Depends payload travels in the wizard answers, and the
+## only upstream file is the stub README marking the tree as
+## intentionally empty.
 ##
 ## Schema: see src/new/templates/mod.rs.
 
@@ -11,4 +14,6 @@ detect:
 build_depends: []
 architecture: all
 rules_dh_line: "dh $@"
-files: []
+files:
+  - path: README
+    template: README.tpl