docs: add memory quality,forgetting and lifecycle RFC

[jiannnnyyyyy]

Which issue or RFC does this PR close?

Closes #1590

RFC: docs/en/rfcs/0000_memory_quality_and_lifecycle.md

Rationale for this change

Issue #1590 discusses how Memory quality signals, lifecycle validity, forgetting, and maintenance should be defined before implementation begins. This PR records the agreed design boundaries and evaluation plan in an English RFC.

What changes are included in this PR?

• Define independent quality dimensions: provenance/verification, importance, novelty, and current-state validity.
• Define deterministic filtering of known inactive and superseded entries.
• Restrict quality reordering to the fixed RRF pool with a maximum ±2 position displacement.
• Add the proposed MemoryRankingTrace without changing public RRF scores or HTTP APIs.
• Define compatibility with the recall gate and optional reranker.
• Define near-duplicate handling, L1 recoverable deactivation, and review-gated L2 maintenance.
• Separate logical lifecycle management from physical deletion and compliance retention.
• Add competitor, research, security, and evaluation references.

This PR contains documentation only. It does not change runtime code, persisted formats, or public APIs.

Are there any user-facing changes?

No released behavior changes. The RFC is a design proposal; implementation remains subject to review and future changes.

No public API, HTTP contract, or persisted-format changes are included.

How was this change tested?

• uv run zensical build --clean --strict
• git diff --check

No runtime tests were added because this PR only adds documentation.

AI usage statement

OpenAI Codex, powered by GPT-5, was used to draft and review the RFC text. The final content was checked against Issue #1590, the repository RFC guidelines, and the existing PowerContext documentation.

[Teingi]

中英文rfc都需要