fix: allow ps resize before an app's first release

[dmonroy]

Summary

• ps resize (and ps list) crashed with could not find DDB item ... DEPLOYSTATUS for any app that had never had a successful release, even though ps resize is explicitly designed to support setting a size before the first deploy.
• Root cause is a regression from Formatting cleanup 🧹 #102 ("Formatting cleanup"), which changed psResizeCmd from silently discarding LoadDeployStatus()'s error to treating it as fatal via checkErr. Before that commit (v4.6.7), a missing DEPLOYSTATUS item never surfaced, so the command fell through to the existing graceful "service does not exist" handling by accident.
• Fix: add a sentinel ErrDDBItemNotFound, and only treat that specific case as non-fatal in LoadDeployStatus (leaving DeployStatus nil), rather than reverting to silently discarding all errors. Real errors (auth, throttling, etc.) still fail loudly, unlike the old v4.6.7 behavior. GetServices gets a nil-guard it needs now that LoadDeployStatus can return a nil DeployStatus without erroring.
• No changes needed to cmd/ps.go — both ps resize and ps (list) already handle a nil DeployStatus gracefully via FindProcess's existing nil-receiver check; this also fixes ps list's identical, longer-standing bug (predates Formatting cleanup 🧹 #102).

Test plan

• go build ./..., go vet ./..., go test ./..., gofmt -l all clean
• New unit test pinning the sentinel error's wrap/unwrap contract
• Verified live against a real AWS account on a genuinely fresh app (zero prior builds): crashes on unpatched v4.7.0, works correctly with this fix
• New behavioral branches (LoadDeployStatus's non-fatal path, GetServices's nil-guard) aren't covered by an automated test — no DynamoDB-mocking infra exists in this repo to exercise them in isolation