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/makefile/manifest.yml

@@ -2,7 +2,11 @@
 ## debhelper's makefile buildsystem runs `make` for the build and
 ## `make install DESTDIR=...` when the Makefile carries an `install:`
 ## target (missing targets are skipped gracefully), so plain `dh $@`
-## plumbing is enough here.
+## plumbing is enough here. The phony-install hint of
+## src/new/templates/makefile.rs (whether dh_auto_install will run
+## `make install` for an existing tree) is the only logic; the skeleton
+## bodies below are static data (the install mapping is rendered for
+## skeletons only, whose phony install target is known by construction).
 ##
 ## Schema: see src/new/templates/mod.rs.
 
@@ -14,4 +18,11 @@ build_depends:
   - build-essential
 architecture: any
 rules_dh_line: "dh $@"
-files: []
+files:
+  - path: hello.c
+    template: hello.c.tpl
+  - path: Makefile
+    template: Makefile.tpl
+  - path: debian/install
+    template: install.tpl
+    skeleton_only: true