Add onboarding intake brief and cast board

[breadback00-dev]

Goal

Reduce first-run confusion by giving players a brief how-to-play orientation and a living map of the people/entities involved in Case Box 001.

Scope

• Add an Intake Brief that explains the investigation loop without turning the case into a manual.
• Add a People & Entities board for Mara, Elian, Porchlight, the resumed account, sponsor layer, Search/Oracle, and the Archive/player.
• Let cast details update as evidence is reviewed.
• Keep the experience grounded in investigation: ordinary human stakes first, myth underneath.
• Verify desktop/mobile layout and collapse behavior in browser.

Done When

• New players can answer what they are supposed to do in the first minute.
• New players can identify the central people/entities without reading external docs.
• The orientation layer does not reveal the canonical answer early.

[breadback00-dev]

Implemented in PR #14 with commit ab5c0a4.

What changed:

• Added a collapsible Intake Brief with the investigation loop: inspect, tag, unlock, submit.
• Added a People & Entities board for Mara, Elian, Porchlight, the resumed account, sponsor layer, Search/Oracle, and the Archive/player.
• Cast cards now update as related evidence is reviewed, giving players social orientation without revealing the canonical answer early.

Verification:

• npm run check
• Python Playwright confirmed intake visibility, collapse/expand behavior, cast board visibility, cast updates after opening Elian evidence, no console/page errors, and no desktop/mobile horizontal overflow.
• Added browser proof screenshots under Case Box 001 - The Creator Who Left But Kept Posting/docs/.