[plan] Refactor NodeLink Shared Filesystem for Agents

_Automatically composed from the parent epic at dispatch time._

9 coders in 3 phases along the Architect's C0–C6 seams. C1 is split into core, wire and doors. C3 owns both accounts-root literals and the cache path. AC5.1 is carried as narrowed by A3. Critic found 2 major issues; both fixed.