Fix #456: connect flow graphs across turns

[willwashburn]

Summary

• The session flow graph now carries the main-rail terminal into the next non-empty turn with a default edge, producing one connected session DAG.
• Cross-turn default edges explicitly represent continuity across an intervening user prompt, while dispatch and return edges remain the independent cross-rail layer.
• Terminal subagent dispatches retain both main-rail continuity and subagent return paths as an intentional acyclic diamond.
• JSON keeps the existing schema and edge kinds; Mermaid and SVG snapshots now render the connected flow.

Validation

• cargo fmt --check
• cargo test --workspace
• BURN_GOLDEN=1 cargo test -p relayburn-cli --test golden -- --nocapture (flow JSON and Mermaid snapshots verified; unrelated pre-existing state-schema snapshot drift remains on origin/main)
• cargo test -p relayburn-cli --test flow_svg -- --nocapture

Fixes #456

🤖 Generated with Claude Code.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 1 additional finding.

[cubic-dev-ai]

All reported issues were addressed across 5 files

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Review details

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Configuration used: Organization UI

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Run ID: 2e7259a3-a87b-46a8-b3ef-115c54207159

📥 Commits

Reviewing files that changed from the base of the PR and between 962b2b7 and d212b0d.

⛔ Files ignored due to path filters (1)

• crates/relayburn-cli/tests/fixtures/flow-svg/flow-session-claude-3turn.svg is excluded by !**/*.svg

📒 Files selected for processing (4)

• CHANGELOG.md
• crates/relayburn-sdk/src/analyze/flow_graph.rs
• tests/fixtures/cli-golden/snapshots/flow-json.stdout.txt
• tests/fixtures/cli-golden/snapshots/flow-mermaid.stdout.txt

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