feat: add cross-rig support to design-to-beads skill

[sfncore]

Summary

• Auto-detects cross-rig mode from file paths in the plan's coverage matrix
• Adds Target Rig field to issue draft template and coverage matrix
• Phase 5 Execute: groups issues by rig, runs bd create from each rig's beads path
• Cross-rig parent-child via bd dep add --type parent-child (--parent flag is same-DB only)
• Cross-rig blocking deps use full prefixed bead IDs
• Completeness review subagent validates rig assignments match file paths
• Report includes per-rig bead ID mapping and distribution

Single-rig projects remain completely unchanged — cross-rig mode is auto-detected only when the coverage matrix shows multiple target rigs.

Companion PRs:

• feat: cross-rig support for beads-creation-expansion gt-toolkit#1 (beads-creation-expansion formula)
• feat: add cross-rig support to epic-delivery skill #1 (epic-delivery skill)

Test plan

• Verify single-rig design-to-beads works unchanged
• Test cross-rig detection from mixed file paths in plan
• Verify Target Rig appears in draft template and coverage matrix
• Test per-rig bd create execution (correct prefix per rig)
• Test cross-rig parent-child wiring with bd dep add
• Verify completeness review catches rig misassignments
• Verify report includes per-rig breakdown

🤖 Generated with Claude Code