feat: project index and the optedIn census scope

[pacphi]

Why

The dashboard must find every managed project (design: Finding every managed project).

In scope

cacheDir()/projects.json as a hint; the census catches missing entries; read-only host records; ak dashboard --project ; the marker .agentic-kit/project.json is the authority.

Out of scope

Opting projects in.

Acceptance criteria

• A deleted index is rebuilt without loss
• A project with a marker but no index entry still appears

Dependencies

Blocked by: #330

Blocks: #356, #367, #387

Plan

• Release: 4.0.0-beta.3
• Size: size:M · Workstream: ws:project-scope
• Epic: feat: project-scope P6, the dashboard for all work (epic) #301
• Card: P6-02 in the card inventory
• Sources: Design (docs(plans): project-scoped management design and its prerequisites #289) Finding every managed project

Verification

• node scripts/run-tests.mjs focus (new index tests)