Add chunked sync for large .archcore/ trees

[ivklgn]

Problem

The sync payload builder sends everything in one request (internal/sync/payload.go), which won't scale to very large .archcore/ trees (10k+ files).

Expected result

Sync batches large payloads into chunks, preserving the "update the manifest only after a confirmed server response" invariant per batch.

Impact

Lets very large knowledge bases sync without exceeding request limits.

Scope

• Batch the payload above a threshold.
• Preserve the manifest-only-on-success invariant per batch.
• Tests with a synthetic large corpus.

Files: internal/sync/payload.go, internal/sync/manifest.go, cmd/sync.go (+ tests)

Acceptance: a 10k+-file .archcore/ syncs in batches without exceeding request limits.

Status: blocked — the sync feature is currently gated off, and this targets a scale not yet observed in practice.