wiki(claude): add session-handoff trigger line + close existing-projects-upgrade

claude-skills/CLAUDE.md gains `session handoff: read on start, write on end`
between `pull remote before work` and `follow project discipline` — mirrors
the canonical project-bootstrap template (v1.12.0) and .admin/CLAUDE.md.

Existing-projects-upgrade partial close (2/7):
- ✓ cwd (this commit)
- ✓ .admin/  (separate repo commit 29724d41, NOT pushed)
- ⏭ .workshop  skip (workspace-contract prose, flat trigger doesn't fit)
- ⏭ victor/books, pilorama98.ru, pilonuxt, OpeItcLoc03/common, board-viewer
  deferred — not on this machine, per-machine upgrade in respective sessions

Closes [session-handoff-existing-projects-upgrade] (partial — pattern mirrors
using-yt-tools-test-trigger split-out).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -7,6 +7,7 @@ use project wiki
 use task management system
 check across all projects
 pull remote before work
+session handoff: read on start, write on end
 follow project discipline
 follow tdd-criteria
 delegate to interns when allowed