fix(sync): validate autosync accepted seqs

[Alan-TheGentleman]

Summary

• validate cloud accepted_seqs cardinality before autosync acks local mutations
• keep acking original local mutation seqs, not cloud-assigned seq values
• add regression coverage for nil/empty/short/long accepted seq responses

Fixes #238
Fixes #298

Tests

• go test ./internal/cloud/autosync ./cmd/engram
• go test ./...

Review

• Fresh reviewer approved: no blockers.

[Copilot]

Pull request overview

This PR tightens autosync’s safety checks when pushing local mutations to Engram Cloud by requiring the server to return an accepted_seqs list whose cardinality matches the pushed batch before acknowledging (acking) local mutation seqs. This prevents autosync from marking local mutations as acked when the server response is missing or structurally inconsistent, addressing reported silent-ack failure modes.

Changes:

• Validate that PushMutations returns a non-nil result and that accepted_seqs count matches the pushed batch size before acking local mutation seqs.
• Ensure autosync continues to ack the original local mutation Seq values (not cloud-assigned seq values).
• Add regression tests covering nil/empty/short/long accepted_seqs responses.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
internal/cloud/autosync/manager.go	Adds result validation and accepted_seqs cardinality checks before acking local mutation seqs.
internal/cloud/autosync/manager_test.go	Adds/updates tests to ensure local seqs are acked only when accepted_seqs cardinality matches, and never acked on mismatch.

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