feat(memory): temporal validity and supersession reasons

[chrishonson]

Summary

Implements INVEST #2 from metacortexplan.md (Temporal Validity / Fact Versioning), including the design-review split of supersession into two semantically distinct reasons:

• changed (default) — the old fact was true of its era (e.g. a job switch). Sets valid_until on the deprecated document; it remains true-of-period for valid-time slices covering that window.
• corrected — the old record was never true (e.g. a mistyped date). Retracts it on the belief axis (supersession_reason: "corrected") without closing a valid-time window, excluding it from valid-time results while keeping it in the audit trail.

Changes:

• MemoryMetadata gains optional valid_from, valid_until (epoch ms), supersession_reason ("changed" | "corrected"), and initiator ("user" | "agent").
• deprecate_context accepts optional supersession_reason (default "changed") and initiator; response payload now includes item.supersession_reason.
• search_context accepts optional valid_at (epoch ms); implemented as a service-layer post-filter after the vector search returns (no new Firestore composite indexes required). A document matches iff (valid_from absent or <= valid_at) AND (valid_until absent or > valid_at) AND supersession_reason !== "corrected".
• remember_context/storeContext accept optional valid_from/valid_until passthrough at the type level (not yet exposed as an MCP input on remember_context, per the agreed scope).
• All new fields are optional — fully backward compatible with existing callers.
• Docs: metacortexplan.md INVEST Add Claude Code GitHub Workflow #2 status → Implemented 2026-07-11; CLAUDE.md tool table and data-flow descriptions updated for the new deprecate_context/search_context contracts.

Test plan

• npm --prefix functions run build — zero errors
• npm --prefix functions test — 56/56 passing, 73.27% overall coverage (≥60% required)
• New unit tests in service.test.ts: default reason + valid_until, corrected does not set valid_until, initiator recorded, valid_at window filtering (inside/before/after), corrected exclusion, absent-fields-always-match
• New end-to-end test in mcp.integration.test.ts: deprecate_context with supersession_reason/initiator over real MCP Streamable HTTP transport, search_context with valid_at for both "corrected" exclusion and "changed" window behavior
• InMemoryMemoryRepository fake mirrors the real FirestoreMemoryRepository.deprecate() behavior exactly

🤖 Generated with Claude Code