burn diagnose: surface execution-graph (relationships + tool-result chronology)

[willwashburn]

Context

PR #77 landed the execution-graph substrate (#42) — SessionRelationshipRecord and ToolResultEventRecord are now persisted by the ledger. 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.

Today burn diagnose <session> (packages/cli/src/commands/diagnose.ts) reads queryAll, queryCompactions, and readContent, then runs attributeWaste + detectPatterns. It never reads the relationship or tool-result-event tables that PR #77 introduced — there's no queryRelationships / queryToolResultEvents call. That makes it impossible to answer the chronology / outcome questions issue #42 was meant to unlock:

• which subagent invocation cost the most?
• which tool calls errored three times before succeeding?
• how many spawned subagents never completed successfully?

The data is in the ledger; burn diagnose just doesn't read it yet.

Proposal

Extend burn diagnose <session> to:

• Query SessionRelationshipRecords and ToolResultEventRecords for the session (and its child / parent sessions where relationship rows make the connection).
• Render a compact relationship summary: root, subagent chains, fork / continuation links if present. Reuse / share the renderer with the subagent-tree consumer migration in this PR's sibling issue.
• Render a tool-result-event timeline: per toolUseId, list the chronological (eventIndex, ts, status, eventSource, contentLength) rows. Highlight cases where a tool call has multiple events with errored statuses (the substrate for retry-loop attribution called out in Execution graph for passive readers: session relationships and tool-result event chronology #42 and Waste-pattern detection: retry loops, consecutive failures, compaction loss, edit-revert #11).
• Roll up terminal-status counts per session (completed / errored / cancelled / unknown) so the top-of-output session totals can include "X of N tool calls errored".
• Add a new burn summary --by-relationship subcommand variant that aggregates totals (turns, cost, tokens) grouped by relationshipType across the queried sessions, mirroring the existing --by-subagent-type.
• Add --json parity for both the diagnose graph view and the summary --by-relationship view.

This issue covers all of burn diagnose's execution-graph surfacing plus burn summary --by-relationship, since they're the same wiring (query the new tables, group, render).

Acceptance criteria

• burn diagnose <session> includes a relationship summary section listing each relationship row with type, related session id, and (when present) subagentType / description / parentToolUseId.
• burn diagnose <session> includes a tool-result chronology section keyed by toolUseId, listing events in eventIndex order with status / event source / content length.
• burn diagnose --json includes relationships and toolResultEvents arrays alongside today's fields.
• burn summary --by-relationship aggregates queried sessions by relationshipType, producing the same shape as today's --by-subagent-type (totals, counts, median / p95 cost where appropriate).
• A session with no relationship / event rows renders today's diagnose output unchanged (graceful empty sections).
• Tests exercise both populated and empty-graph sessions.

Out of scope

• burn waste --patterns reuse of ToolResultEventRecords — separate follow-up under Waste-pattern detection: retry loops, consecutive failures, compaction loss, edit-revert #11 (closed) or a new issue if waste-pattern detectors want to consume the graph.
• Migrating the existing --subagent-tree consumer onto SessionRelationshipRecord — covered by the sibling subagent-tree issue spun out of Execution graph substrate: SessionRelationshipRecord + ToolResultEventRecord (#42) #77.
• New ledger query functions beyond queryRelationships / queryToolResultEvents (which PR Execution graph substrate: SessionRelationshipRecord + ToolResultEventRecord (#42) #77 already exports).

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)