Exclude .claude, .agents and AGENTS.md from git in the new project

After the templates are copied, create-project.sh adds /.claude, /.agents and
/AGENTS.md to .git/info/exclude of the new project as its own step, "Git
excludes". No .gitignore or tracked file changes and nothing is committed.
The step is skipped when the framework steps are skipped, and a path git
already tracks stays tracked and is named in the step.

The .git/info/exclude code of exclude_env_file moves into the shared helper
exclude_from_git (git.sh), which both now use; the .env behavior is unchanged.

Refs #66
Task: MIL-009#2

src/lib/envfile.sh

@@ -26,19 +26,8 @@ env_file_key_list() {
 # file: the entry goes into .git/info/exclude, which is never committed. It
 # does nothing when .env is already ignored.
 exclude_env_file() {
-  local dir="$1" gitdir exclude
-  if git_project "$dir" check-ignore -q -- "$ENV_FILE_NAME"; then
-    return 0
-  fi
-  gitdir="$(git_project "$dir" rev-parse --absolute-git-dir)"
-  exclude="$gitdir/info/exclude"
-  mkdir -p -- "$gitdir/info"
-  # Start on a fresh line when the file does not end with one.
-  if [[ -s $exclude && -n "$(tail -c 1 -- "$exclude")" ]]; then
-    printf '\n' >>"$exclude"
-  fi
-  printf '%s\n' "# RepoFoundry: the credentials file of this project" "$ENV_FILE_NAME" >>"$exclude"
-  git_project "$dir" check-ignore -q -- "$ENV_FILE_NAME" ||
+  local dir="$1"
+  exclude_from_git "$dir" "RepoFoundry: the credentials file of this project" "$ENV_FILE_NAME" ||
     die "could not make git ignore $ENV_FILE_NAME in $dir; nothing was written to it"
 }