docs: add memory manager documentation

[opieter-aws]

Description

Documents the long-term memory feature (MemoryManager) for the TypeScript SDK. The implementation shipped across #2544 (memory manager), the Bedrock KB store + extraction work, and #2631 (injection), but had no user-facing documentation. Agents that should remember a user across sessions had a supported API and no page explaining it.

Adds two pages under site/, both TypeScript-only (the feature is TS-only today):

• Memory (concepts/agents/memory.mdx) covers the MemoryManager: the MemoryStore interface and multi-store routing, the search_memory / add_memory tools, programmatic search/add, automatic extraction (triggers and ModelExtractor), and context injection. It opens by distinguishing memory from the adjacent session-management and conversation-management features, since the three are easy to conflate.
• Bedrock Knowledge Base Store (concepts/agents/bedrock-knowledge-base.mdx) documents the vended BedrockKnowledgeBaseStore: the split connection/store config, CUSTOM vs S3 vs OTHER writability, search/ingestion, scope-based namespacing, and the IAM permissions a writable store needs.

Code examples live in runnable, type-checked snippet files (*.ts plus sibling *_imports.ts) and are pulled into the pages via --8<-- includes, so every rendered block carries its imports and is copy-paste runnable.

This PR also adds a memory tag to site/src/config/tags.yml and reconciles the terminology lock (.agents/references/terminology.md): "memory" was previously listed only as a banned synonym for session management, which collides with the shipped MemoryManager feature name. The lock now carves out "long-term memory" as the canonical term for the feature, distinct from session persistence.

Related Issues

#2544, #2631

Documentation PR

This PR is the documentation. New pages registered in site/src/config/navigation.yml under Concepts.

Type of Change

Documentation update

Testing

Built the docs site against the SDK source (astro build): both pages render, all --8<-- snippet regions resolve, and the broken-links checker passes across all pages. The snippet sources type-check against the SDK via the site/test-snippets harness (tsc --noEmit). Reviewed both drafts with the docs-writer and docs-reviewer voice/terminology passes.

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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