--state-dir flag overriding APS_HOME

[0xLeif]

Part of GOAL.md workstream A.

Per-command --state-dir for commands that touch persisted state (get, set, watch, dump, reset). Wins over APS_HOME.

Acceptance criteria

• Flag accepted on all state-touching commands
• Precedence: --state-dir > APS_HOME > ~/.aps
• Invalid/unwritable path produces a clear non-zero error
• smoke.sh exercises the override (and can then run fully isolated, see smoke cleanup issue)

[0xLeif]

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