APS_HOME environment-based state root

[0xLeif]

Part of GOAL.md workstream A.

Today FileState always lands in ~/.aps (APSPaths.configure()). Add APS_HOME so agents and tests can isolate state without touching the real home directory.

Acceptance criteria

• APS_HOME set: FileState root becomes $APS_HOME (note.json lives directly under it)
• Unset: current ~/.aps behavior unchanged
• Precedence documented for the upcoming --state-dir flag (flag wins over env)
• README documents the variable

[0xLeif]

Shipped in PR #2 (aps 0.2.0, merge commit 62198af). Verified in review: gates green, behavior confirmed locally.