Migration docs: FEATURE-PARITY.md and ORIGINAL-FUNCTIONALITY.md added alongside ARCHITECTURE.md rewrites

The docs side of the AWS migration spans 23 commits. The README is rewritten around the new architecture (CloudFront/S3, API Gateway/Lambda, AgentCore/Strands, Cognito Identity Pool, Bedrock). ARCHITECTURE.md lands at the repo root and is updated several times as the migration progresses - Cognito User Pool diagrams, direct S3 upload flow, MCP/Skills additions, dead table removal.

ORIGINAL-FUNCTIONALITY.md catalogs every feature of the upstream codebase with a status column for the migration branch. Notable gaps it documents: DELETE /user/account requires Cognito AdminDeleteUser (not done at time of writing), the tabular model selector still referenced Gemini models in the UI, and dead userSettings.ts code that read claude_api_key/gemini_api_key from the database needed cleaning up.

FEATURE-PARITY.md maps every API endpoint with migration status and notes. Chat streaming moved from a backend route to direct AgentCore invocation; document upload was pending a refactor to presigned PUT URLs. A Known Gaps section covers drag-and-drop attachment rendering and sharing limitations. The To-Test section lists manual verification steps; by the end of this commit range, most core paths show verified status.

The README and architecture diagram diverge somewhat over the course of the commits - the diagram captures a moment in time and is already partly stale by the final commit. The commit messages are more reliable than the diagram when they disagree.