feat: add cross-rig support to epic-delivery skill

[sfncore]

Summary

• Auto-detects cross-rig mode from mixed bead prefixes in leaf beads
• Creates integration branches per rig (each rig's refinery merges independently)
• Dispatches leaves to their owning rig based on prefix → routes.jsonl mapping
• Monitors all rigs' integration statuses each check cycle
• Runs quality gates per rig on each integration branch
• Handoff messages include full cross-rig state (rig map, per-rig progress)
• Plan-vs-actual summary includes per-rig breakdown

Single-rig projects remain completely unchanged — cross-rig mode is auto-detected only when leaf beads have mixed prefixes.

Companion to Xexr/gt-toolkit#1 (cross-rig support for beads-creation-expansion formula).

Test plan

• Verify single-rig epic delivery works unchanged
• Test cross-rig detection with mixed-prefix leaf beads
• Verify per-rig integration branch creation
• Test cross-rig dispatch (beads slung to correct rig)
• Test cross-rig monitoring (all rigs checked each cycle)
• Test per-rig quality gate execution
• Verify handoff preserves cross-rig state

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