feat(telemetry): add export command

[EhabY]

Summary

• add the coder.exportTelemetry command UI flow
• register the command through CommandManager and package contributions
• connect date-range selection, format selection, save dialog, export progress, and telemetry flushing

Closes #903.
Follow-up: #952 tracks adding recent local telemetry to support bundles.

Stack: 4 / 4. Base: #961.

Review follow-up

• adds TelemetryService.flush()
• flushes buffered local telemetry before discovering and reading telemetry files so recent events are included

Validation

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

Generated by Coder Agents.