feat: `git mind status` dashboard command (WATCHTOWER)

[flyingrobots]

Summary

Implement git mind status — a single command showing graph health and composition.

Deliverables

• Node summary: total count, breakdown by prefix (count + percentage)
• Edge summary: total count, breakdown by type
• Health indicators: blocked items, low-confidence edges (< 0.5), orphan nodes (0 edges)
• --json flag for CI pipeline consumption
• Test suite covering all summary computations

Context

WATCHTOWER milestone (depends on BEDROCK, which is complete). This is the first consumer of the node query API shipped in #184.

Part of the TECH-PLAN roadmap §9.4.