docs(external-memory): document external memory backend (ARCHITECTURE, STRUCTURE, CONFIGURATION, README)

Adds the external-memory-backend feature to root docs:

- ARCHITECTURE.md: new 'External-memory flow' subsection under Data Flow
  (tee-on-write → 3-slice recall + dedup + persist-before-settle → m[1]
  <external-memory> render → W2 correctives → ctx_search external source);
  new 'External memory backend (Hindsight)' Key Abstraction; cached_m0_
  external_recall_hash marker added to the schema-migrations note
  (deliberately NOT a mustMaterialize trigger); project-config trust
  boundary updated to include memory.external in the user-only strip
  list; unified search entry updated with the external source.

- STRUCTURE.md: new external-memory files listed under src/features/
  plus a Key File Locations block (engine-agnostic provider,
  Hindsight impl, recall orchestrator, snapshot read, W2 hooks,
  project-security strip) and the Pi mirror path
  (@magic-context/core re-export).

- CONFIGURATION.md: full memory.external block (provider, endpoint,
  api_key, project_bank template, main_bank, retain_sources, tags,
  recall sub-block) and the memory.external.recall defaults; full
  failure-semantics + cross-harness note; user-config-only security
  callout; example updated.

- README.md: concise mention in the Recall section + a new callout
  for long-term memory; external added to the four-source ctx_search
  description and the agent-tools table.

Author: Tehan