v3.1.0 — Project stacks convention

Adds a project-stacks convention to the managed Claude Code context: a way
to group related repos on a dev server so a fresh Claude session can orient
itself without re-exploring the code.

What's new

• ## Project stacks section in the managed ~/CLAUDE.md block, describing
  stacks at ~/Repositories/_stacks/<name>/ (a README.md, a sessions/
  history, and a tasks/ kanban board).
• A worked example template at /opt/vps-setup/templates/project-stack/ to
  copy when creating a real stack.

Cheap session orientation

Session start no longer bulk-reads the whole backlog. README.md, the latest
~10 sessions/, and tasks/doing/ are read in full; tasks/open/ is scanned
one line per task via a frontmatter grep, opening individual tasks only when
the work touches them.

Upgrading

vps_update re-renders the managed CLAUDE.md block automatically — no new
migration. Operator content below the end marker is preserved.