fix: prevent false forget receipts and redaction resurrection

[NeerajCodz]

Motivation

• The runtime forget_memory path reported successful receipts for derivative backends it never touched, which could mislead callers into believing redaction/deletion had been propagated.
• The consolidate_session flow re-ingested full session transcripts with a deterministic ID, allowing previously redacted or soft-deleted memories to be resurrected as active plaintext records.
• Used the rust-best-practices skill to ensure changes are minimal and idiomatic.

Description

• Updated forget_memory in src/runtime/governance.rs to keep the canonical postgres receipt as ok but mark derivative backend receipts (qdrant, neo4j, redis, s3) with status: "not_implemented" so the API no longer falsely claims propagation.
• Hardened consolidate_session in src/runtime/consolidation.rs to compute the deterministic consolidation memory_id before ingestion and return Err(CoreError::InvalidInput) if an existing memory with that ID exists and its status is not Active, preventing re-creation of forgotten/redacted memories.
• Changes are limited to src/runtime/governance.rs and src/runtime/consolidation.rs and preserve existing behavior where applicable while removing misleading receipts and blocking unsafe re-ingestion.

Testing

• Ran cargo test -p nextral --lib --quiet and all library tests passed (11 passed, 0 failed).

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Codex Task