fix(memory): expose reasons for skipped empty-URI operations

[yeshion23333]

Summary

• classify policy- and validation-driven empty-URI memory operations with stable reason codes
• return intentionally skipped operations as scoped skipped_operations in commit task results and include reason codes in logs
• persist skipped operations in memory_diff.json with a summary.total_skipped count for archive-level auditing
• keep deletes in the same streaming apply batch as unresolved upserts, allowing delete execution to be suppressed when an upsert is skipped
• reuse the existing peer-memory policy without adding config, LLM calls, or telemetry

Scope

• policy and validation skips are treated as expected skips and do not fail the commit task
• unexplained empty URIs keep the existing behavior: they remain internal MemoryUpdateResult.errors and are not promoted to task errors, failed operations, or memory_diff.json skipped records
• memory_diff.json keeps skipped operations as top-level audit metadata; its operations section continues to contain only applied file changes
• delete suppression applies to file deletion only; it does not make replacement processing atomic or change the existing ordering of replacement link migration

Compatibility

• normal URI resolution and writes are unchanged
• peer_memory_enabled propagates the existing MemoryPolicy.peer_enabled decision and is not a new configuration option
• the new parameter is appended to the public extraction signature so existing positional callers keep their parameter bindings

Testing

• targeted tests cover reason classification, streaming batch isolation, compressor and commit-task propagation, memory diff generation and merging, and legacy positional compatibility
• memory-diff regression tests pass (17 passed); ruff check, format checks, and git diff --check pass for the touched files

Related to #4131