Implement first history and artifact UI/API cut

[terisuke]

Scope

Implement the first product cut for Issue #27 and Issue #28 without claiming the full Phase C surface.

This issue is satisfied when:

• saved style guides, sessions, and completed briefs are available through read APIs;
• the web app exposes a history picker for reopening saved workflow state;
• style guides and article briefs render as human-readable cards;
• ADR 0002 and validation docs record implemented and remaining scope;
• focused handler/static UI tests pass.

Out of scope

• add-persona authoring UI;
• broad edit persistence beyond existing fork-on-edit/session/brief save paths;
• project/article/draft version browsing;
• browser E2E under Issue Add browser E2E coverage for model config and question CRUD #13.

[terisuke]

Implemented and merged in PR #82. Validation is recorded in docs/validation/issue-27-28-history-artifacts-2026-05-03.md. #27/#28 remain open for the broader Phase C scope documented in ADR 0002.