feat(telemetry): add streaming JSON export writer

[EhabY]

Summary

• Add writeJsonArrayExport that streams telemetry events to disk as a JSON array. Chunks are piped through stream/promises pipeline so memory stays flat regardless of maxTotalBytes (default 100 MB, no enforced ceiling).
• Extract a shared src/util/fs.ts module hosting tempFilePath, renameWithRetry, and a new writeAtomically(path, write) helper that captures the temp-file + rename + best-effort cleanup pattern.
• The new helper replaces CliCredentialManager.atomicWriteFile (16 lines -> 4) and backs the export writer. SSH config / support bundle / CLI manager keep their bespoke flows and just update imports.

Refs #903.

Stack: 2 / 4. Base: #959. Next: #961.

Why streaming

Export ranges include "All time", which reads everything on disk. With maxFileBytes and maxTotalBytes user-tunable and unbounded, a buffered writer would peak at ~2x the export size in memory and OOM at high caps. Streaming holds memory constant at ~12 lines of code vs ~10 for the buffered version.

Changes

• src/telemetry/export/writers.ts: writeJsonArrayExport returns the event count; uses serializeTelemetryEvent from the shared wire format module so the output cannot drift from the on-disk shape.
• src/util/fs.ts (new): tempFilePath, renameWithRetry, writeAtomically.
• src/util.ts: drops the moved helpers.
• src/core/cliCredentialManager.ts: atomicWriteFile now defers to writeAtomically.
• test/unit/util/fs.test.ts (new): suite for the three helpers, including atomic-write success, partial-write rollback, and callback return values.
• test/unit/telemetry/export/writers.test.ts: switched to memfs, shared createTelemetryEventFactory, and the shared serializeTelemetryEvent. Added a test that a mid-stream failure leaves the destination untouched.
• test/unit/util.test.ts: moved-out blocks removed.
• test/unit/core/supportBundleLogs.test.ts: vi.mock target retargeted to @/util/fs.