@W-22167805@ Add trace session utilities (listSessionTraces, readSessionTrace, readTurnIndex)

[WillieRuemmele]

Work Item

@W-22167805@

Summary

Adds utility functions to the agents library for local trace file access, supporting the sf agent trace commands in plugin-agent.

Changes

• listSessionTraces(agentId, sessionId) — lists all trace files for a session with metadata (planId, path, size, mtime)
• readSessionTrace(agentId, sessionId, planId) — reads a single trace file by planId, returns PlannerResponse | null
• readTurnIndex(agentId, sessionId) — reads turn-index.json for a session, returns TurnIndex | null
• Exports PlannerResponse, PlanStep, FunctionStep, TurnIndex, TurnIndexEntry types (previously unexported)

Files Changed

• src/utils.ts
• src/index.ts

Test Plan

• npm run build passes cleanly
• All functions exercised via esmock stubs in plugin-agent consumer tests
• Consumed by plugin-agent PR (see dependent PR)

[EstebanRomero84]

Looks good. Can you add UTs for the new utility functions?