feat: Session Handoff Files (.continue-here.md)

[dean0x]

Summary

Explicit handoff files with structured sections (current_state, completed_work, remaining_work, decisions_made, blockers, next_action) created on pause, deleted on resume. YAML frontmatter for machine parsing.

Context

Identified in Anvil competitive analysis (#114). Note: WORKING-MEMORY.md already provides automatic session continuity, making this partially redundant.

Effort

Small