feat: add analyze-graph for execution-context recommendations

[dapi]

Summary

• add the read-only analyze-graph --handoff FILE command
• normalize explicit typed handoff nodes and relations without semantic inference
• report advisory context recommendations, high fan-out nodes, repeated co-access, and unresolved or weak evidence
• provide Markdown and versioned JSON output with contributing nodes, relation types, and source references
• add focused fixtures and update CLI architecture documentation

Verification

• go test -count=1 -race ./...
• go vet ./...
• git diff --check
• successful CLI build and JSON smoke test for the high-fan-out fixture

This PR addresses GH-48 without changing handoff or Memory Bank files.