feat: claude-memory-sync — back up Claude Code auto-memory to its own private repo

Auto-memory (~/.claude/projects/*/memory, 774 files across 49 projects) goes to
gitea vh/claude-memory, not this repo. The git dir is ~/claude-memory.git and its
work tree is ~/.claude/projects, and exclude rules admit only memory/ folders, so
transcripts stay local and nothing is added inside ~/.claude.

- home_root/.local/bin/claude-memory-sync: commit, rebase onto origin, push, under
  a flock; --status; --init for a fresh machine. Every run records its result
  for the drift check.
- home_config/systemd/user/claude-memory-sync.{service,timer}: hourly with
  Persistent=true. Linked but not yet enabled.
- link-dotfiles --check now also reports uncommitted changes (a stale session
  can write old content back through an intact link) and a failed or stale
  (>3h) memory sync.
- CLAUDE.md and README document the backup and the never-put-credentials-in-memory rule.

home_config/systemd/user/claude-memory-sync.timer

@@ -0,0 +1,10 @@
+[Unit]
+Description=Hourly Claude Code auto-memory backup
+
+[Timer]
+OnCalendar=hourly
+RandomizedDelaySec=5min
+Persistent=true
+
+[Install]
+WantedBy=timers.target