fix: complete Neptune and Postgres state cutover

[jonathanhaaswriter]

Summary

• remove local snapshot read fallbacks so graph reads stay on the live/configured runtime path
• migrate app-state persistence and Snowflake backfill paths to Postgres while preserving the legacy Snowflake migration source
• sync generated config env docs and stabilize time-windowed graph intelligence API tests

Validation

• env GOFLAGS=-mod=vendor go -C "/Users/jonathan/Documents/cerebro-worktrees/writer-main-neptune-pg-cutover-20260330" test ./internal/app ./internal/appstate ./internal/agents ./internal/findings ./internal/api ./internal/cli ./internal/snowflake ./internal/remediation ./scripts/generate_agent_sdk_packages -count=1
• python3 scripts/devex.py run --mode changed --base-ref writer/main
• python3 scripts/devex.py run --mode pr --base-ref writer/main