SessionStart memory index: name the repo's project in the lead-in #756

Problem

Agents do not know which project the cwd repo belongs to, so they cannot target project on save_memory. The SessionStart memory index only tags entries that are already project-scoped ([project: slug]); nothing says "this repo is in project X".

Change

• At SessionStart, fetch the repo's confirmed projects from the server (kcap-server AI-2470 decides the endpoint shape; the index body stays a bare array for older CLIs).
• Render a lead-in line in the memory fragment, for example: This repo belongs to project "capacitor". Save learnings that span its repos with project: "capacitor". One line per project when the repo is in several.
• Fail-open like the rest of the index fetch: a missing or failed call emits no lead-in.
• MemoryIndexEmitter keeps the existing [project: slug] annotation on entries.

Depends on

kcap-server AI-2470 (part of AI-2468).