docs: Add STATE.scm for AI session context persistence

Implements Guile Scheme stateful context system per hyperpolymath/state.scm
guidance. Captures:
- Current position (v6.2.0 released, v7.0 at 85% completion)
- MVP route to v7.0 release (Q4 2025)
- Known issues (test coverage gap, single maintainer)
- Questions for maintainer (5 decision points)
- Long-term roadmap (v7.0-v8.0, 2025-2026)
- Critical next actions (prioritized task list)

Enables context persistence across AI assistant sessions.

Author: claude