docs: surface soft-delete and demote patterns via update_episode

[jjrawlins]

Summary

Agents can already expire episodes via update_episode but the current description — "Update metadata, tags, or expiration of an episode" — doesn't hint at the cleanup use case. This documents the two agent-safe patterns:

• Soft-delete via expired_at past timestamp (reversible — clear expired_at to restore)
• Demote via a deprecated tag (visible but filterable at query time)

Follows the "no delete_episode MCP tool" design from #16/#18 — permanent deletion remains a deliberate human action outside the agent's toolkit.

Changes (docs only, no behavior changes)

• internal/mcp/server.go: update_episode tool description and its tags / expired_at property descriptions now mention soft-delete and demote use cases
• internal/api/openapi.go: PUT /api/v1/memory/episodes/{id} description matches
• README.md: new "Cleanup Patterns" section with MCP examples for soft-delete, demote, and scheduled expiration

Test plan

• go build ./... — compiles
• just test — all existing tests pass (no test changes needed, docs-only)
• Live test: served /openapi.json reflects the new description
• Live test: update_episode with past expired_at hides the episode from default search and listing (both REST and MCP)
• Live test: soft-deleted episodes remain in the store (episode count unchanged, recoverable)