fix: resolve all golangci-lint errors failing CI

[xfiive]

Summary

• Fix 35 lint errors across 23 files that caused CI failures on dev and feat/ci-feedback-loop branches
• All errors were from the golangci-lint step (errcheck, unused, gosimple, staticcheck, ineffassign)
• Net result: 41 insertions, 174 deletions — mostly dead code removal

Changes by category

errcheck — unchecked error returns (6 fixes)

• internal/sqlexec/executor.go — tx.Rollback wrapped in deferred closure with _ =
• internal/updates/cache.go — os.WriteFile best-effort, ignored with _ =
• internal/updates/checker.go — SaveNotificationData (2 occurrences)
• internal/orchestration/auth_validator.go — json.Encode
• internal/orchestration/event_handler.go — PlanSpinner.Stop() (3 occurrences)

unused — dead code removal (10 fixes)

• Removed unused functions: stringListToBulletItems, assertViewNotContains, showEndpointIndicators, isTUIOutputMode, startInlineSpinner
• Removed unused struct fields: transitionOn/Target/Data, skippedCount, eventHandler, workerPool
• Removed unused vars: errPromptCancelled, errPromptTimeout

gosimple — code simplifications (7 fixes)

• Simplified nil+len check to just len() check
• Used type conversions instead of struct literals (TableDoneMsg)
• Used switch msg := msg.(type) to eliminate redundant type assertions
• Replaced fmt.Sprintf("literal") with string literals

staticcheck (5 fixes)

• Replaced nil context with context.TODO() in tests
• Replaced deprecated grpc.DialContext + grpc.WithBlock with grpc.NewClient
• Fixed unused variable assignments

ineffassign (1 fix)

• Fixed ineffectual assignment in DSN parser

Test plan

• go build ./... passes
• go build -tags dev ./... passes
• CI pipeline passes (golangci-lint, tests)