feat: sd triage — PageRank + betweenness + critical path ranking

[RogerNavelsaker]

Summary

• Adds src/graph.ts with pure graph algorithms: reversed PageRank, Brandes betweenness centrality, and critical path DFS
• Adds src/commands/triage.ts — sd triage command that ranks ready issues by composite graph score
• Registers triage in src/index.ts
• Colocated tests in src/graph.test.ts (6 cases: empty, single node, linear chain, diamond bottleneck, score bounds, external edge filtering)

How it works

sd triage applies the same eligibility filter as sd ready (open issues with all blockers closed), then ranks them by:

score = 0.5 × PageRank + 0.3 × betweenness + 0.2 × criticalPathLength

• PageRank runs on the reversed dep graph so nodes that unblock important work rank high (authority flows from dependents back to blockers)
• Betweenness centrality (Brandes algorithm) identifies coordination bottlenecks — issues that sit on many shortest dependency paths
• Critical path length — longest forward chain of blocks edges via DFS

Human output shows score (pts), critical path depth, and betweenness per issue. --json emits a structured envelope for agent consumption.

Test plan

• bun test src/graph.test.ts — all 6 graph algorithm tests pass
• sd triage in a repo with a dependency graph returns issues ranked by score
• sd triage --limit 3 returns top 3
• sd triage --json emits valid JSON with success, issues, count
• sd ready behaviour is unchanged

Closes #6

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