feat(telemetry): add export event reader

[EhabY]

Summary

• add telemetry export range helpers for preset and custom UTC date ranges
• add filename-based telemetry JSONL discovery and streaming event reader
• validate stored telemetry JSONL with Zod and convert it to the exporter event shape

Refs #903.

Stack: 1 / 4. Next: #960.

Review follow-up

• validates stored timestamps with z.iso.datetime({ offset: true })
• conservatively includes the day after a selected range so buffered events flushed after UTC midnight are not missed
• keeps exact filtering at the per-event timestamp level
• removes unused duplicate stored-event types

Validation

• pnpm test:extension ./test/unit/telemetry/export/range.test.ts ./test/unit/telemetry/export/files.test.ts
• pnpm typecheck
• pnpm lint
• pnpm format:check

Generated by Coder Agents.