v0.16.0 — templates can include repo files via include()

Added

• Templates can include repo files: {{ include("path/from/repo/root") }}.
  The motivating case is the AGENTS.md fan-in (one shared "hub" file linked
  into many agent config dirs): a tool that reads exactly one file and has no
  import syntax previously forced a choice between linking the hub verbatim
  or forking it — and a fork silently stops receiving hub edits. A .tmpl
  source can now compose hub + delta ({{ include("agents/AGENTS.md") }}
  followed by tool-specific lines), rendered into the staged file the target
  links. Include semantics are deliberately narrow: the argument must be a
  safe repo-relative fragment (no .., no leading /), must not live under
  .stitch/ or .git/, must reach a regular file without passing through a
  symlink (one hop only, the same rule as add --source), and must be valid
  UTF-8. Included text is embedded verbatim — never rendered — so recursion
  cannot occur and .tmpl arguments are rejected outright. Fresh renders
  re-read included files on every apply/diff/doctor, so editing an
  included hub shows up as a content diff and a doctor staging-drift
  warning until the next apply converges it.