burn summary --subagent-tree: consume SessionRelationshipRecord (replace TurnRecord-only path)

[willwashburn]

Context

burn summary --subagent-tree <session> already ships (PR #64, closed #8) but reconstructs the tree solely from TurnRecord.subagent.{agentId,parentAgentId,parentToolUseId,subagentType,description} — see packages/cli/src/commands/summary.ts:30-38, 167-174 and packages/analyze/src/subagent-tree.test.ts.

PR #77 landed a normalized SessionRelationshipRecord substrate exactly so this view doesn't have to keep reconstructing relationships from per-turn fields. From the PR's deferred-work list:

Consumer CLI surface — burn summary --subagent-tree, burn diagnose, burn waste --patterns, burn summary --by-relationship. The execution graph is now persisted, so these can be built on top.

Issue #42's acceptance also explicitly calls this out:

The graph is rich enough that #8 can consume it instead of reconstructing everything from isSidechain and parentUuid alone.

Today the ledger persists relationship rows (via appendRelationships / queryRelationships from PR #77) but the subagent-tree CLI never reads them. That keeps Codex / OpenCode subagent trees blocked on each parser duplicating the per-turn field population, and means the tree command can't show fork / continuation / subagent-of-subagent chains the new graph captures.

Proposal

Migrate burn summary --subagent-tree (and the aggregateBySubagent / subagent-tree builder in @relayburn/analyze) to build the tree from SessionRelationshipRecords (joined to TurnRecord cost data), with TurnRecord.subagent retained as a fallback for sessions ingested before the graph was populated.

• @relayburn/analyze gains a builder that takes (turns, relationships) and produces the same tree shape consumers already render.
• The renderer continues to roll up cost / token totals per node from the underlying TurnRecords, joining child turns via agentId.
• When relationships is empty for a session (legacy ingest), fall back to today's TurnRecord.subagent-only path so existing data still renders.
• Surface the four relationshipTypes (root / subagent / fork / continuation) in the rendered output, not just subagents — at minimum, an annotation when a node's relationship type is non-subagent.

Acceptance criteria

• burn summary --subagent-tree <session> produces identical output for a Claude session whose SessionRelationshipRecords are present (vs the legacy TurnRecord.subagent-only path) on a fixture that has both populated.
• A Codex or OpenCode session that has SessionRelationshipRecords populated (per Codex passive reader: populate SessionRelationshipRecord and ToolResultEventRecord #87 / OpenCode passive reader: populate SessionRelationshipRecord and ToolResultEventRecord #93) renders a subagent tree without per-source CLI branches.
• A session that has only TurnRecord.subagent (no relationship rows) still renders correctly via the fallback.
• subagent-tree.test.ts is extended to cover the new graph-based builder; the legacy fallback retains its existing tests.
• --json output includes the underlying relationshipType per node.

Out of scope

• New burn diagnose --execution-graph / burn summary --by-relationship surfaces — separate follow-up.
• Re-attribution of existing ledger lines (no rewriting of historical TurnRecords).
• Codex / OpenCode parser population — covered by Codex passive reader: populate SessionRelationshipRecord and ToolResultEventRecord #87 / OpenCode passive reader: populate SessionRelationshipRecord and ToolResultEventRecord #93.

Refs

• Spun out of Execution graph substrate: SessionRelationshipRecord + ToolResultEventRecord (#42) #77 (Execution graph for passive readers: session relationships and tool-result event chronology #42 follow-up)