feat: work-bead-to-work-bead cross-plan dependency mapping

[pofallon]

Summary

Enable fine-grained work-bead-to-work-bead dependency wiring across flight plans, allowing more parallelism than epic-level blocking.

Background

Currently, cross-plan dependencies are epic-to-epic only (Phase 1-2). This means if plan B depends on plan A, ALL of plan B's beads are blocked until ALL of plan A's beads complete. In practice, only specific beads in plan B may depend on specific beads in plan A.

Requirements

Dependency Declaration

• Extend depends_on in WorkUnit model to support cross-plan references using <plan-name>/<work-unit-id> syntax (e.g., add-auth/setup-jwt-middleware)
• Validate that referenced plans exist and referenced work unit IDs are valid
• Support both intra-plan (existing depends_on) and cross-plan references

Resolution & Wiring

• During refuel, resolve cross-plan work unit references to bd_ids by:
  1. Finding the epic by flight_plan_name state metadata
  2. Finding the child bead matching the work unit ID
• Wire work-bead-to-work-bead dependencies via bd dep add

Auto-Detection (Stretch)

• Use Phase 3 file-scope overlap data to suggest work-bead-level dependencies
• The Recon agent could suggest depends_on entries for the decomposition agent

Key Files

• src/maverick/flight/models.py — extend WorkUnit.depends_on validator for cross-plan syntax
• src/maverick/library/actions/cross_plan_deps.py — add work-bead resolution
• src/maverick/workflows/refuel_maverick/workflow.py — wire work-bead deps after create_beads

Acceptance Criteria

• WorkUnit depends_on accepts <plan>/<unit-id> cross-plan references
• Cross-plan work unit references resolve to bd_ids during refuel
• Work-bead-to-work-bead dependencies are wired via bd dep add
• Validation rejects invalid cross-plan references (unknown plan, unknown unit)
• Tests cover cross-plan resolution, wiring, and error cases
• Backward compatible — existing intra-plan depends_on still works

Depends On

• feat: file-scope overlap detection for cross-plan dependencies (Phase 3) #88 (Phase 3 file-scope overlap detection)