feat: add solstone user-wide skill for cross-project journal queries
New SKILL.md installs via `npx skills add` to ~/.claude/skills/solstone/,
giving any AI agent outside the solstone project a read-only query interface
to the journal — recall, who, today, transcript, people, and status.
Makefile updated to include user-wide skill install in `make install`.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>